For beginners, the biggest way to make a difference early on is by becoming your squad's medic. It doesn't take a mountain of twitchy reflexes to start slinging around first aid kits and patch up your squadmates - and you'll make lots of new friends in the process. After gaining just 40 career points (two or three games for most people), you'll get your first rank increase, and with it, a new piece of gear to equip.
If you've enjoyed healing wounded buddies so far, you are going to love what comes next: being able to bounce back teammates from death. Spend your first unlock on the defibrillator - the first item available in the Assault class tree. Reanimating dead troopers not only confuses the enemy by sending back the same soldiers that they just killed, but personally, you'll get just as many points as if you slayed an opponent. Also, if you happen upon an unsuspecting enemy, hit him with the paddles for an embarrassing kill - it's a terrible way to go.