Really the biggest taboo I can warn you about is be careful if you go Vanguard. Using Charge + Nova might seem like a powerful combo, but it'll only work for the first couple rounds in a match. More often than not you'll end up getting yourself killed far away from your team, making it a pain to try and get to you to ressurect you.Schmaleop said:
Also be a team player. Make sure that you're paying attention during the Objective Waves so that you can help out with them.
As for class/weapon suggestions, it depends on your play-style. Most classes either have a focus on powers or a focus on their weapon damage. For instance, most of the infiltrator characters will want to focus on increasing their Sniper Rifle damage and go for one-shot kills every time. The best rifles are the Widow, Black Widow, and the Javelin (this is my opinion, obviously) but don't let the Mantis fool you, it's got some very respectable power as well. And given that it's the base-form of the sniper rifle, chances are you'll level it up pretty quickly.
As for leveling up your characters, play bronze until you're about lvl 10, then Silver for the rest. Gold and Platinum are very tough and I'd only suggest it if you've got a party of friends that know what they're doing. Random groups don't tend to fair well in them. Really there is no "fast track" for character leveling seeing as how no matter what, everyone gets the same amount of exp after a match so getting kills doesn't matter. The Volus Engineer, for instance, is an entirely support-based class...pretty much a medic. He might not get a lot of kills, but the fact that he can keep everyone's shields up is a big help.
Some fun classes with the starting lineup are the Asari Adept and the Female Quarian Engineer. The Biotic Explosions that the adept can make are very powerful, and the freeze-burn combo the engineer has is pretty sweet too. With classes that can do more damage by focusing on their powers, you'll want as fast a cooldown as you can get, so most people just rock out with a single weapon. If you manage to get yourself a Carnifex pistol, then that's a great weapon to give your "caster" classes. Packs a good punch when it's needed and keeps your cooldown very quick.
The Turian Sentinel can be pretty badass if you use him right. Having Overload and Warp is a VERY good skill-set, Overload is arguably one of the best powers in the game since you can spam it and keep multiple enemies stun-locked. The drawback is that the Turian Sentinel can't dodge...at all, so you have to be careful with him. Put points into his special armor just to have some added defense, but you'll definitely want to focus on Warp and Overload, those are his bread'n'butter.
As for the packs, it depends on what you're looking for. Premium Spectres are good if you're hunting for weapons, the Reserves is good if you're looking for characters. Probably the most useful, though is the Jumbo Equipment Pack. You'll wanna keep yourself fairly stocked up on the items you use with the d-pad, and that's the best way to get those items.