My question is aimed at death match games, such as Halo and Call of Duty. Assume a high degree of comfort with single-player FPSs, but no experience with multiplayer.
Play with a buddy, clan or quad who is/are better than you and can show you the ropes. Halo 3, in particular, had some great clans who were interested in helping newbies out.
In team games, USE YOUR MICROPHONE. Communication is key in team games so you'll want to use the mic to coordinate your assaults and call out enemy positions, especially snipers.
Pay attention to the gametype you are playing. A lot of people play EVERY game mode like it's free-for-all or team deathmatch. If you are playing an objective game like capture the flag or territories, going for a good k/d ratio is not the best use of your time. Granted, killing the other team can give you advantages, but you should still be spending the lion's share of your time working towards the actual goals of the game.
On team modes, learn to be a good team member. For example, in a capture the flag game, you should either be a part of the group going for the enemy flag or a part of the group defending your flag. You shouldn't just be running all over the map by yourself. In a game like Battlefield Bad Company 2, you'll want to remember your support functions and help out by dropping ammo and medkits, throwing sensors and repairing vehicles for your team.
If you can play private matches, set up a private game and investigate the maps. Learn where the weapon drops, objectives, etc. are located and the paths between them. If you can play local private vs. games, hook up a second controller and check out the sniper spots and hidey holes to get an idea of how hidden people are and what it looks like when someone is camping there.
See if the game has a good wiki or MP FAQ on gamefaqs or the web. A lot of people will share great tips for succeeding in MP. You can also check out the paper guides (Prima, etc.), but those are usually not as good as their online counterparts and aren't updated to reflect new maps and game patches.
Take some time to become familiar with the weapons. You'll die a lot while doing this, but try to get a feel for their effective ranges, reload times, clip size, power and so on. Try to think about what sort of weapons will work better on which maps. Are there vehicles? Long sniper lines? Lots of close quarters?
If the game has classes, try all of them out and give each of them some play time. Even if you hate a class (for example, I'm not a fan of sniping), you'll learn the strengths and weaknesses of that class by playing it for a bit.
If the game has a an unlock system, pay attention to completing the unlock requirements once you have the basics of the game down. In some games, the higher-level unlocks can give you a significant advantage on the battlefield.
Learn to shoot for the head. In most games, a headshot does a lot more damage than a body shot. You'll want to reflexively put your reticule on their helmet before pulling the trigger.
Be prepared. Make sure your health and shields are topped off before running out for more mayhem. Find a safe place and reload after each battle. Make sure your ammo and supplies are topped off.
Don't be predictable. Unlike single player, humans are pretty good at figuring out your patterns. Try to mix it up a bit, both with weapon selection and where you roam on the map.
Pick one or two games and practice them as often as you can. More frequent shorter sessions are probably better than fewer longer sessions, though you'll want each session to be long enough to "get into the groove" of the game.
Don't get discouraged. In any game there will be a lot of people with way more skill than you, even with "skill-based" matchmaking. Try to learn from your deaths and view them as a learning experience. Understand that, when switching games, your skills may not transfer well - you'll have to learn new maps and the controls, game modes and weapons will be different.
In order of importance:
- Continue playing. You can't get better if you don't play.
- Learn the game mechanics
- When you die, make a note of why you died. Learning from your mistakes is crucial.
- Watch YouTube videos
- Read up on strategies
- Learn the terrain
- Watch what other people do - both the good and bad, and learn from it
- Try different classes/characters/weapons to see what you are best at, which suits you best
I've never really played Halo (gasp) so I'm not sure if this tip applies to that series... But in CoD you should always, always, always watch the entire kill cam. I've learned hiding places, tactics, loadouts, etc from simply watching how someone destroyed me. Even if it seemed obvious how you died it can still be instructive to watch it from their point of view.
Plus it can sometimes show you how a player made it to a glitched spot so you can subsequently harass them into giving it up and stop being cheap.
Be brazen. Single-player mode teaches you to peak around corners and be generally very cautious; peaking around corners in multiplayer generally means getting your head blown off by a sniper who has been camping that spot for the last ten minutes. As counter-intuitive as it is, it's often safer to sprint to cover through an open field then to peak around corners and try to sneak around.
A lot of these answers are great, but they don't really address the question. Since you say "a high degree of comfort" with single-player FPS, I assume you already know things like mouse look and circle-strafe.
If you want to get good at MULTIPLAYER FPSs, you definitely have to recognize that your squadmates are humans, and internalize that to the point where you are an actual TEAM. This means the skills you need to work on are communication, tactics and keeping morale high.
Communication is essential - if you're a silent lurker, you're essentially the same as an AI bot except more unpredictable, and basically just looking out for yourself. But if you communicate and work together, that's where bigger tactics can work out. Think of a tactic that only a bunch of thinking humans can pull off when working together, like "You, me and Jack go left to draw their fire, we'll use grenades but stay alive and pull back while ninja-boy sneaks in from the right to steal the flag". If a team doesn't communicate and try tactics like that, then it's just a bunch of people playing single-player, but they happen to be on the same map as part of a so-called "team". Get that mic and start talking about intentions ("gonna try reaching the scorpion tank", passing information ("sniper on left cliff!"), and encourage your mates when something doesn't work as planned ("no worries, good try on that one, they got lucky one guy saw us").
Tactics - every game has different ways to be played, and thus different group tactics to apply so it's hard to generalize. You gotta learn and try it out. Different classes do different things as an attempt to force people to play differently by doing different things - if you're in a specific class, understand how that class helps your teammates best. For team deathmatch games, tactics might involve different weapon combinations or different ways to cover an area, or setting ambushes up, flushing out campers, providing cover fire, etc.
Morale - this will probably sound really cheesy, but this is a big one, actually. When a team is demoralized, they're much more prone to losing, all other things being equal. You don't have to be an artificially chippy cheerleader, but at least be aware of when your teammates are getting frustrated, and encourage them to brush it off, or focus on something different. Also, when something goes well, congratulate everyone ("way to go!"), or offer praise for something done really well ("nice headshot on the last guard"), for instance.
For good examples of this, you should actually watch someone take part in a large WOW raid - in order for a high-level raid to go well, people have to coordinate and help each other. Watch for the kinds of communication, tactics and encouragement they give each other. A big, failed raid is a substantial loss of time and effort, and so you see the players coordinating carefully to be successful.
One of the biggest things in FPS games: never try to win a "fair fight" unless you have no choice. ALWAYS try to sneak up on the enemy or use a better weapon than they have.
This is the #1 mistake I see noobs make. They think it's honorable or something to try and go toe-to-toe with somebody. Don't do it. When you see someone and they see you, get out of sight and re-engage only when you're sure you have the jump.
You will learn aim and circle-strafing and sniping and all sorts of advanced strategies in time, but at the beginning, just follow that simple rule and your kdr will go up.
Play a lot and train your reflexes. Even if you die a lot, try to improve your aiming. For instance, instead of shooting straight ahead, take a little time and re-aim your crosshair. You are probably dead by then, but with time you'll be blazing fast and shoot at the right moment.
Also, put your melee key at close reach to your moving arrows. Some like it on the mouse, some on Alt, some on E (near W). Like that you can knife people very fast when they're close or you're out of ammo.
Ah yes, when you're running out of ammo, change weapons quickly or use melee. Don't waste time reloading unless you're sure nobody's gonna kill you.
For PC games, one easy way to make your aim better is to use a lower resolution. It makes your opponents bigger on-screen and easier to hit. You may lose some peripheral vision but for me it has been a worthwhile trade-off.
For example, playing Natural Selection 2, a game where I find it challenging to hit other players, I recently switched from a 1920X1200 resolution down to a 800X600 resolution and it is much easier to hit my opponents since they are much bigger on-screen. The game isn't as pretty as before, but I'm now much more competitive and get more kills.