FA is an incredibly deep game to learn, and it will take time and commitment to improve your skills. If you venture online after only playing singleplayer, you'll likely feel overwhelmed. However, there is a lot of information out there that you can use to quickly get up to speed and hit the ground running in multiplayer!
Stuff you should have
These are things that most of us already have and swear by:
FAF client: http://www.faforever.com/?page_id=2 - if you haven't installed it yet, get it from there. This is the main lobby client and is required to play on FAF!
GAZ_UI: http://forums.gaspowered.com/viewtopic.php?p=414706 - widely considered the best UI mod, it adds lots of subtle (and not so subtle!) tweaks to improve the UI's performance and usability. It's a UI mod, so it won't desync the game while playing online.
Open ports: http://forums.gaspowered.com/viewtopic.php?t=2531 - Port 6112 needs to be open, both on your firewall and your router, to ensure connectivity problems don't occur (if you see nils, you probably don't have your ports open!). This page contains a wealth of troubleshooting information to help you get those open first.
Overview of game modes
There are a metric ton of different game modes and various settings you can play with, so here's a quick primer on the most popular ones:
Assassination: The most commonly played, bread-and-butter type victory condition. Kill the other team's ACUs to win.
Supremacy: Not commonly played in normal games; usually used in Survival type game modes in case an "accident" happens to your ACU. You must kill every unit and structure of the enemy for them to officially die.
Share Until Death: If this option is turned on (and it usually is, generally unless the host specifically states otherwise), all of that player's units are killed when their ACU dies. This means that killing ACUs is the most important thing to do; if it's off, killing an ACU can often be detrimental to the team as the enemy team now has a supercharged eco from the dead player's units and structures.
FFA: A Free For All has no set teams, although they typically result in players being mobbed in order of how dangerous they look.
LABwars: A custom game mod, where you can only build LABs! It's a zany game mode with its own special blend of strategy.
Phantom: Another custom game mod, thoroughly described in this thread: viewtopic.php?f=2&t=471
Murder Party: Sort of like a mix of FFA and Phantom: viewtopic.php?f=25&t=300
Survival: Special maps are designated as survival games. In these, you and your teammates fight against the AI in a scripted scenario.
Guides to playing
All of these guides are excellent, and contain a wealth of information on how to play. The first guide is especially useful, and something that everyone should take the time to read.
http://forums.gaspowered.com/viewtopic.php?f=2&t=50272
Useful links
You'll probably want to bookmark these as they're terrific resources:
FAF Wiki: http://www.faforever.com/mediawiki/index.php/Main_Page
Unit Database: http://www.faforever.com/faf/unitsDB/
Forums: index.php
Other cool things you'll want to know
A quick sampling of answers to commonly asked questions:
How can I play an online game?
There are two types of games: Ranked and Custom. Ranked games are purely 1v1, while Custom games can have any number (1-12) of players with any mods or rules the host sets. Custom games are typically more laid back, while 1v1s provide an excellent challenge for sharpening your skills. Try both!
To play a ranked game, simply click on the "Ladder Game" tab, select your preferred faction, and hit Search! You'll be matched up with whomever else is searching at the same time, and be placed on a random map to do battle on. Ranked activity fluctuates, so if you find yourself waiting for a few minutes to get a game, try letting others in chat know that there's someone searching!
To play a custom game, you have two more options: joining a game that's already being hosted, or hosting your own. Joining a game is probably the better option for new players, as more experienced players will be there to guide you. Don't be shy! The community is, on the whole, an extremely friendly and supportive group.
To join or host a game, click on the "Custom Game" tab and either select a game to join, or click "host game" and enter the game's name.
How do I watch replays?
You can double click on any crossed swords that appear next to someone's name in chat. This will allow you to observe a game in near real time (there is a 1 minute delay to prevent abuse of the system for cheating). You can also double click on the space where the swords usually are to view that player's last played game.
Finally, you can paste this in chat, or any other channel, to act as a link to that player's latest livereplay:
livereplay:GPGNET://91.229.20.50/PLAYER_NAME.SCFAReplay
PLAYER_NAME is case-sensitive; for a working example:
livereplay:GPGNET://91.229.20.50/Isen.SCFAReplay
What about this score next to my name?
For the most part, ignore that while you're still learning. It's your global ranking, computed via TrueSkill. What that means is that it's a (semi-accurate) representation of your skill relative to other players. Remember that all of us started out new once, and don't try to play purely for score - just play to improve your skills, and it will naturally rise over time as you get better!
Everyone calls me noob and doesn't want to play with me!
Unfortunately, some people in the community really dislike playing with new players. Or maybe it was an arranged team game that you didn't know about, and were unceremoniously kicked from it. Whatever the reason, it sucks!
My best advice is to hone your skills in ranked first. No one will care about you being a noob (they'll think, "ooh, free points!" ) and you'll be able to test your skills against a wide variety of skill levels.
Some people offer training as well - ask around in chat and I'm sure you'll find someone willing to lend a helping hand.
Lastly, you can always host your own game, or try and find games with similarly skilled players.