This question is too broad and basic, but there are a number of ways monotize your game (see but does ROTMG make $$$? section).
1) Sell it on your website / via Steam or in an appstore like the Apple App Store or Google Play
2) In-app purchases (managed by Apple App or Google Play)
The world of in-app purchases is huge. And what you can sell depends on what kind of game it is .. is it a persistant MMO type world? Then have permadeath and sell scrolls of resurrection in the store. Is it a shooter? Then sell the ability to "unlock" the better weapons. The list goes on, what you can sell as in-app really depends on what your game about.