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.
If you want more medals, here's how to get them:
Group medals: The whole team gets these
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
Melee kills: 5/10/15
Biotic Kills: 10/25/50
Grab enemies from cover 1/3/5