How to obtain experience

You get experience by playing the game.  After every match, you will earn a bit of experience.  If you won that game, you would get more experience than if you lose the game.  There is a cap on how much you experience you can get from any one event but the cap is fairly high and should not be a problem.

What to do with experience

You can’t spend experience and it just stays with you.  the more experience you collect the higher your level.  Three is not really a purpose for experience except to track how much you play.