For every game you win, you will get 300 coins.
If you wish, you can purchase various coin packs in the game Shop. To access it, select the icon represented by a store, in the lower right corner of your screen.
Once you enter the Shop, you'll see your coin and PowerUp balance on the upper part of the screen, with their corresponding icons. If you scroll down, you'll find several PowerUp packs, which you'll be able to buy with coins.
If you scroll further down, you'll be able to buy coins and coin packs. Bear in mind that you can only get coins in the Shop by using real money.