If you have designed, developed and published a game and now want to earn money from it, here are some monetization strategies:

Advertising
You can put banner ads in your game or you can have reward ads which give rewards to users if they watch the ads, you can also have video ads in between game rounds. But make sure that there are minimal ads and they don't disrupt the gaming experience of users.

Merch
You can sell virtual merch like character skins and outfits. Or you can sell game objects like virtual gadgets and stuff. This is a great way to make money from your game especially if the game is multiplayer.

In App Purchases
IAP is when users spend real money for game items like new clothes, character upgrades, new content, etc. But make sure that there is enough content for users to use without paying too because otherwise the game will feel like a money draining app. Also, the users should feel satisfied with the content/objects which they get for the money that they give.

Subscriptions
Here, you can let users buy subscriptions to premium content, early access, etc. This is also a great way to monetize.

Let us know in the comments if there are any other monetization strategies which you have for games..