How can I be a better (multi-)player

So, I almost always end up at the bottom of the standing. I have fewer medals than everyone and just generally do poorer. What are some good techniques to improve my skills in the multiplayer arena?

I play as Infiltrator, but general tips would be more helpful than specific class tips (though mentioning them wouldn't hurt).

Work as a team and supplement your allies' weaknesses. You're an Infiltrator, stay back and hit the hard-hitting enemies, and pick off weaker ones or more distant enemies. Use your tactical cloak to revive hurt allies.

If you're a Vanguard, play hit and run with Biotic Charge (remember it charges your Barrier too). Thanks to Charge you can take a beating, so get in the thick of things and distract some enemies. Melees are surprisingly effective against many shielded targets, especially Cerberus. Stay away from big or armored targets as you're not so good against them.

The other classes play more standardly so I won't go into detail. Just use your powers to back up your teammates, but keep yourself alive. If one class isn't working, don't be afraid to try another. Infiltrator is one of the hardest classes to play right since you have to be able to aim, plus the default sniper rifle is kind of crap.

More generally, try to help your teammates out; if you can burn/freeze enemies to help team members finish off a target, do it. Assists can give you as many points as kills (or more!), the XP you get for a kill is based on how much damage you to to the target before it dies, so don't worry about who kills what, just take enemies down.

Always revive allies when you have the chance, your team is severely crippled even with just one or two members down, and difficulty does NOT scale with number of players.

Medals

If you want more medals, here's how to get them:

Group medals: The whole team gets these

Unknown Location
Unknown Enemy
Bronze/Silver/Gold Mission (awarded based on mission level, IF you finish wave 10)
Full Evacuation: Escape wave 11 with all players alive Partial Evacuation: Escape wave 11 with at least two players alive but not all
Survive rounds without being downed: 3/5/10

Individual medals:

Melee kills: 5/10/15
Assists: 10/25/50
Biotic Kills: 10/25/50
Kills 25/50/75 Headshots 5/10/15
Revives 5/10/15
Grab enemies from cover 1/3/5

Killing enemies is the fastest way to get points, and doing lots of damage to enemies you don't kill also helps.

As an infiltrator, you should invest in your tactical cloak mastery. You can almost double your damage at rank 6 with a headshot, and a good sniper shot from cloak can take out half the shields of an Atlas.

Cloak cloak cloak. It stops you from being shot at (don't earn points while dead) lets you line up your shots better (more hits, faster kills, more points), and gives you a huge bonus damage when specced properly (1 shot all the t hings!).

Also, you should look into capping the objectives during the objectives round. Each of the 4 terminals is 650 points, and you can run over there and start while cloaked, which is good for your team (they can keep everything else distracted), and you (you get all the points).

one of the best ways I've found to get high kill counts is soldier class spec'd for cerberus (unsure if reapers also affected by setup). Make concussive shot your go to power, while taking the rank that makes the damage last over time on organic enemies. the fastest recharge speed, the added damage, the automatic rifle fire and grenades gives high offensive output, enough that when maxed out I had 3 of 5 matches at one point with 75+ kills.

My friend swears by vanguards as well. Will tell you that an aggressive and properly equipped player can force the action, rack up kills and jump huge areas on the map as long as line of sight is unimpeded, which is basically as good as sending a human sized bullet at some mid grade sniper ranges. Personally find it too cumbersome unless you have proper backup such as an infiltrator to revive or assist. Unless you are willing to risk success of a mission in a few seconds, not my advice.

Adept is my preferred class, but you're not guaranteed the points as much. Good thing is that warp affects armor, barrier and health regen, bad news is you're screwed as much against shields and even guardians unless you have their backs or are good enough with a pistol or smg to mail slot them. Crowd control plays are only useful at choke points, shockwave's stagger is good even against brutes and atlas when maxed. being able to use powers around corners is useful for staying alive, and also recharge speed is key in defending.