like Phoenix said, use the pistol, aim for the head. don't be too stingy with grenades. The plasma pistol isn't too bad if you don't have anything better, but do try and find something useful. BTW, a headshot with the sniper is a one-hit kill. Headshots with the pistol take two. Unlike other shooters, so know when to charge in and when to duck and cover.
Otherwise, practice up with a friend or sibling, as multiplayer is much different than single. If you lack either of these
you're a loser go to
http://www.xbconnect.com and download their program. It simulates LAN play over the internet. Its a great (free) alternative to Xbox Live but can lag.