Start as either assault or medic.
With assault you should get easy kills with the GL and the medic has one of the better starting weapons, once you unlock the paddles you can rack up XP quite quickly. Unlocking the Seiga shotgun makes the engineer and recon classes a lot more playable until you unlock a decent SMG and get used to sniping. One of the best anti-armour tactics in the game is to use a shotty equipped Recon to plant C4 on an enemy tank, or booby trap a building.
Try using C4 on a quad bike then driving into an enemy tank.
Try using C4 on a UAV then flying it to an MTAC and detonating it.
As an engineer class, support your armour, use your rockets to help kill enemy armour, rep your friendly tank, and shoot enemy engineers.
Vehicle bonus's are stackable, If you have +vehicle damage and someone else gets in a tank with +vehicle armour you get both powerups. I dont think you get double damage though.
As recon with a sniper rifle, headshots gain a marksman XP bonus that can be significant.
Try counter-sniping with MG's in 3-round bursts, dont forget about bullet drop!
A door or fence blocking your path? Knife it!
Plant C4 at an unguarded Mtac before planting a bomb on it, detonate when the defenders try to disarm it.
Anti-tank mines tend to be more anti-everything.
mortar strikes attack a very small area.
Flank, out-maneuver and sneak. Some of my best kill streaks have come from hiding after loosing both MTACS then sneaking up behind squads (especially groups of snipers) and knifing the lot of them.
When attacking an enemy squad kill the medic first, a dead medic cant revive anyone and revived soldiers add there tags back into the pool.