As a fairly new player myself, I have a few things that can help as well.
Get the S-AMS? system for the sunderer, and pop a sundy whenever you're driving with a big group. Tanks may seem attractive, but if you drop a sunderer somewhere even vaguely useful, you can get a decent chunk of xp from it. Granted, its only 2xp a spawn, but if all the other sundy's go down, suddenly the entire zerg is respawning out of yours. Plus, you can repair it when it inevitibly takes a rocket or brick of C4.
Speaking of Sundy's, ANY vehicle weapon is going to be better then your gun you are carrying. Granted, on a deployed sundy, you're an easy target, but even with the basic basilick gun, you can deal damage to tanks, and potentially shoot down planes. If the person has improved the guns, they are VASTLY superior. Just remember, if you can't aim below the horizon, which may be canted if they park on a hill, its an anti air gun, and if you can't aim very high, its anti ground. I mention sundy's in particular because they are often parked with no one gunning, but if you see a plane coming, hop in and shoot at it. For every certain amount of damage you do to planes, you get xp.
Get used to MAXes. Your basic max, with an anti infantry gun in one hand and an anti vehicle in the other, is quite useful against infantry, though not quite as good against either as it would be with two dedicated weapons. However, your anti infantry gun keeps things pinned and does constant damage, and your AV weapon is fairly capable of finishing off anything near the blast point. Sadly, the Burster AA weapon is not quite as good against infantry, and I'm not sure why the rounds don't blow up near tanks aside from balance, but if you must have two rapid fire weapons as a max, it can work. Just, don't shoot at vehicles, you draw attention to yourself and won't do anything.
As a MAX, save your charge for when you need to get OUT of combat. There is ALWAYS fighting ahead of you, and you will eventually reach it, even plodding along, but when you hit half life, being able to turn and scram can keep your suit alive to rejoin the fight. Likewise, if you do die, it MAY be prudent to wait a bit before respawning, especially if you see a medic icon near you.
Pick either medic or engineer to start with. Yes, its annoying being regulated to support, but neither of them are slackers in killing. In fact, the engineer probably has the most possible ways of killing things, and the most potential variety of attacks. Sadly, it does pay for that, in that it can't QUITE be as good as the dedicated bearers of such tools, but the sheer variety means that you are very unpredictable.
Keep an eye out for cross class unlocks. For example, the battle rifles are shared between engi and heavy. Unlocking it on one with certs means that they both get the rifle, and a battle rifle turns both classes into a cut rate infiltrator, but with expanded options. Likewise, I think they share C4 as well, as I don't remember unlocking it my heavy.
Certain things are great for certs, others aren't. Killing is actually rather poor for certs. Reviving? Amazing. Repairing vehicles? Great. I HIGHLY recommend getting the revive grenades. They are expensive, but can VERY quickly pay for themselves in short order. See those 10 guys who just ran out of the spawn room into the crossfire of 20 tanks. Throw a nade at them, rearm at the terminal for a new grenade, and get ready to throw it again as they proceed to die in the crossfire again.
If you go medic, healing is secondary to your main job. If you have the choice between healing someone and reviving someone, always revive, even if you have to go out of your way. Fortunately, with your class power, you can do both at the same time, but healing is almost never worth it with the medic gun. Likewise, healing grenades aren't worth it either.
Biolabs are possibly the best source of certs, assuming there are two zergs in there. With 30+ people on both sides fighting for a lab, you can pretty much just tuck in as a medic and start ressing. If you are defending, just figure out where they enemy is coming from, stay out of the way, and res. On the offence, once again, just stay out of the way, preferably tucked away safely in the teleporter room, and again spam rezzes.