In-app Currency

How can I set up something like a shop where a player can buy "extras"
I don't want for them to have to use real money to pay, but rather have them earn coins inside of the game.

 

It's a pretty generic question, so here's a pretty generic answer.

Your game controller (a high-level object that stays persistent between scenes) can have a currency variable that gets added to whenever the player gains coins. When interfacing with your store UI, check against and deduct from that variable for purchases.

Not sure how much more detail to go into without knowing what exactly you're having issues with and how much you have already.

 

I

I don't have anything right now.
So how can I go about creating that?

 

From this and your other posts, it sound like you don't have much experience with Unity in general. Go through the tutorials at http://unity3d.com/learn and start with the basics before you worry about an in-game store. Start small and work your way up, but keep your game idea in mind while you learn and think how what you're doing can be applied to what you WANT to do.

 

We offer a full shop solution for real money & virtual currency products on the Asset Store, here:
$30: https://www.assetstore.unity3d.com/en/#!/content/12343
$10: https://www.assetstore.unity3d.com/en/#!/content/22628

You should still get familiar with Unity and follow some tutorials in the learn section, as others suggested, though (judging by your very popular post "How can I add a ceiling").