Getting Online
The beauty of Civ III:Conquests is there is no need to download any program nor set up accounts so getting online is easy. Simply click Multiplayer game and then click Online and your in.
The Lobby
There are two ways of occupying the lobby, either by using the game or by using Gamespy (GS). Gamespy is a very useful lobby tool it allows you to private message (PM) other Gamespy users, check what players are currently in the lobby, mute annoying players, see what players are currently starting games and add friendly players to buddy lists. Conquests lobby is very simple allowing you to see players games, start and join games, connect directly by IP, or switch connection to LAN.
Joining a game
Joining a game is easy. Simply click on any open game and enter the players room. Often players may wait to collect a certain amount of players so patience in getting a game setup is important.
Ladder games
We’ve all been here. You join a game and the host asks you if you are ladder. You reply no and you are asked to leave 
. What is a ladder? Why can’t a new player play in a ladder? Are they elitist? No. The ladder is a league of online multiplayers who use win/loss reports to keep players from quitting games due to bad land, no resources, etc. If a ladder player allows you into their game he/she has no guarantee that you will not quit. It’s free and easy to sign up and you find games are more rewarding using this style of play.
The Setup
Ok now its time to decide what sort of game you will play. Multiplayers have broaden their horizons quit a bit so simply choosing ancient start games are a thing of the past. Now there are several choices like Quick Civ, Medieval start, Industrious Start, Modern Start, Future Start, UU Madness (All civs are capable of building any UU depending on what resources, strategic and luxury, they hold…for instance anyone can build a Jaguar Warrior if you hold Wine.) , One city challenge, CTON, Team Games, FFA, etc. A couple things you can count on for any game is that they game will use Simultaneous Moves and accelerated production. Both are important for making a game as short and fun as possible.
The Civ
Ok we have decided to play the most popular online style; an ancient game, acc production, pangea, one city elimination normal point system and 90 turns. Ancient era starts will usually never get far beyond early medieval techs…perhaps one or two techs, so now we must decide on what civ we should chose. Which is the most powerful? First lets see what sort of traits will be useful in the ancient era.
Agricultural: Basically the dominant trait in the ancient era. Players without this trait will have a hard time keeping up with players who do. Its ability to grant an extra food per turn for each city built near a fresh water terrain square adds up over 90 turns. Also it’s ability to make use of desert is a huge advantage of being able to overcome poor starts. Drawbacks are if you find yourself without a river or lake you basically now have a one trait civ.
Industrious: Still a popular trait even though it was somewhat hamstringed from its glory in the PTW days. Still over time this trait is dominant over non-industrious civs. Drawbacks are workers are cheap and a non-industrious civ can overcome their shortcomings by simply increasing its workers, also since it is rare for civs in a 90 turn game to switch governments you do not receive your shield bonus.
Religious: Cheap temples = more points = good. Drawbacks are that rarely does anyone change governments so the 1 turn anarchy is lost.
Militaristic: Cheap barracks, faster unit promotions, and a higher percent of gaining a military leader all excellent benefits. Drawbacks are in acc production barracks can be population rushed with one shield for any civ.
Expansionist: The gamblers trait. This can be the most powerful or most worthless trait in the game. Expansionist is good for getting an early feel for the terrain, quickly finding enemies, find strategic resources, pop early settlers, techs, and friends. Drawbacks are if you find your self on an island, in a huge jungle and goody huts only gives you maps or conscript warriors then your trait has become worthless.
Commercial: This traits main purpose in Ancient start is getting an early b-line for philosophy since it starts with Alpha. Drawbacks are since it takes awhile to develop your cities the gold bonus is short lived and the reduced corruption is almost unnoticeable.
Scientific: Cheap libraries, free techs, and increased SGL chance…nuff said. Drawbacks are if you find yourself in a gold crunch you may have to shut off your research and thus losing any advance you have.
Seafaring: Early naval 4 movement is very powerful along with extra commerce for coastal cities. Drawbacks are that Seafaring isn’t paired up with a good civ combination for the ancient era. Carthage and Netherlands are decent civs but Carthage without being agricultural and with an expensive UU often falls behind in development while Netherlands UU comes far too late.
Ok now we have gone over the traits lets look at what civs bring it together the best. For time sake I will only go over the 8 most popular civ choices for online multiplayer games.
Sumeria A spear for the price of a warrior is an incredible advantage…what makes it worse for sumeria’s enemies is this civ is also agricultural. This is thee most dominant civ in Multiplayerdom and is often the first civ to be selected. They are impervious to early jag or chasqui rushes and make excellent choking instruments. The strategy with this civ is to expand like crazy…build nothing but workers, settlers, enkidu and archers. With Sumeria you will have 20 cities while other players have 10. Drawbacks are inability to chose when to have your Golden Age (GA). A player can attack your enkidu while you have one or two city giving you a weak GA and basically ruin your production for the rest of the game.
Aztecs Not the unstoppable early threat they once were but still the second best civ in the game. Cheap barracks and fast growth can in the right hands make this civ a powerhouse. There are varying strategies to use the Aztecs, you can build a handful of Jaguars Warriors and go for an early kill (remember all you need is to take one city), or sit back and use your Jaguar Warriors for a sentinel net then trigger your GA in a timely manner. Drawbacks are early GA’s and the Jaguar Warrior becomes an obsolete military weapon eventually.
Celts Can you say Gallics? With the decrease in cost of the Gallic Swordsman the Celts are now one of the most powerful civs in Conquests. With proper development and the correct resource you can utterly dominate a game. The popular strategy with this civ is to play defensively early, build cities, develop your land in preparation for a mid game GA. Drawbacks are if you find yourself without iron you have a toothless civ.
Iroquois A cheap three attack 2 movement UU is nothing to sneeze at. Iroq is often played in the same manner as the Celts. The commercial trait gives you a free shot at philosophy. Drawbacks are without horse you are also gumming your food and MountedWarrior defense is only a 1 and a stack of archers and warriors can make your life a living hell.
Inca A lot of debate over the usefulness of this civ however it is often the third most popular civ. The traits are what make this civ and the ability to chose either an early or later GA. The drawbacks are the expensive and weak traits of the Chasqui Scout and inability to build cheaper scouts. Not to mention the expansionist trait runs its course after about 40 turns.
Zulu The Impi is a terrifying foe if he finds you unprepared. The militaristic trait and the retreat ability of the impi means you’ll be seeing plenty of elite units . The drawbacks are this civ is not agricultural so he will not be able to keep up with the other civs in development. If zulu finds himself without a close nieghbor he has lost his advantage entirely.
Maya This is one of the most underrated civs in the multiplayer world. Often near last to be picked due to its near worthless UU it more than makes up for it in its traits. Increased growth and faster production means you can out build ANYONE. The drawbacks is as mentioned its weak UU. So what that means is you have to rely on ancient units like horsemen or swordsmen which both fall short of either MWs or Gallics.
Egypt O how the mighty have fallen. Once the coveted civ of MPers it is now a trash can pick. For Egyptian players it is a must to attack a neighbor as soon as possible since attempting to out develop an agricultural civ is futile. Drawbacks is without horse you are worthless. You will not be able to keep up with your agricultural neighbors and all you can do is hope that other civs will be killed before they come for you 
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Strategies
For this I will simply reference a friends strategy article here.
Building the Mega Stack
So you wanna learn how to build 20, 40, 80, 160 unit stacks? Well I’m about to unveil some secrets of some seasoned veteran MPers.
First thing you need to build the Mega stack is an agricultural civ. The extra food is a must in order to get your city sizes up. You will also need fresh water to plant subsequent cities in order to get their sizes up or b-line for aqueducts. For the mega stack player (MSP) early granaries are a must and should be the only improvement he builds for the time being. For now the MSP is not interested in warfare and should build a good sentinel net capable of detecting any invading army.
From these granary cities the MSP will build early workers and settlers…his goal is by 1700-1500 b.c. to have 10-15 or more cities and all workable tiles improved with mines on grassland and irrigation on plains and desert. Don’t bother mining hills or mtns since you will not receive any bonus in GA…also do not bother mining or irrigating bonus grassland since again in a despotism you do not receive any bonus for improving this land during a GA (In team games in which you have research help it is possible to reach monarchy but it is rarely done unless you are religious).
At about 2000 b.c. build a barracks in your capital and in one or two other cities and hook up your resource and begin to build warriors or chariots (depending on which unit you are looking to upgrade). Cut off research and save money. With well worked tiles and high city sizes you should be able to save a good sum of money. At or near 1700 b.c begin building barracks and temples in all your cities and by 1500 b.c. you should trigger your GA. If you did this right (and it will take some practice) You should be able to upgrade 10-20 units and begin building 5-10 units a turn. After 10 turns you should have near 80 to a 100 units. Go forth and destroy .