Some say, it is to capture control points.
Edit:
Here I will tell you how to get fast experience and money. You started as a infantry soldier, keep playing with him till you get a high rank, high rank soldiers have bigger salary. Remember not to change to another career! Why? You will earn knowledge about the game, knowledge about maps, units etc. Also having an infantry soldier gives you the ability to play almost every battle. (tank crewman and fighter pilots have longer queue times) Choose the path to unlock a sub(small) machine gun or a light machine gun, don't buy modifications in the beginning of the game. But why? Because some of them are very expensive to fix! Doing that you will be able to fill up your pockets with a lot of credits, you can hit your first million very soon. After that you acquire a new soldier that can become tank crewman, recon or pilot, without chance to becoming penniless (you have the infantryman aka "cash cow" paying the bills for other soldiers). For example your tank repair cost can raise sky high and battle profits fall, in this situation it is good to have another credit income. What gives more experience in the game? Capture a point (+100 xp per capture), Defend a point (+60 xp per kill), destroy planes, destroy tanks. This are common ones, and some others. Push with your teammates, move to the conquer area to capture this point as soon as possible (ASAP). Always, always get inside a jeep, only when there's no transport pick up a bike or a village truck. Late in your career you will probably have three or four soldiers like: Infantryman, Tank Crewman, Paratrooper and a Fighter pilot. In some matches you will be able use all of then, so you can fill in any empty slot in your team doing the difference!
Excerpt from the official New Players' Guide