Well unfortunately a new player will not have many upgrades that others do. With this in mind the assault class might be a good start, as you can use the grenade launcher to instantly kill many players that you would otherwise lose a shootout with (due to bad starting rifles, magnum ammo, etc).
edit: I forgot to mention, if you pick assault, as soon as you get the ammo pack item, spam them everywhere. Many times as soon as I spawn I switch to the pack and just toss it at a group of people. Helps out engies and snipers a lot and gets you much needed points.