How do I make a shop where you get hats with in-game cash ?

Something without using robux, only In-game cash.

This should help you make a shop:

Start by giving Players a Money variable: https://wiki.roblox.com/index.php?title=Leaderboards

Second make a Buy button: https://wiki.roblox.com/index.php?title=API:Class/ClickDetector

Third Check if the Player has enough money: https://wiki.roblox.com/index.php?title=Conditional_statement

Here is an example of checking if the Player has enough money:

Player.Money.Value >= 100 then
	print("Player has enough Money!")
end

Then to give the Player the hat, just set the Hat's Parent to the Player's character. (Just in case you don't know about Children and Parents): https://wiki.roblox.com/index.php?title=Creating_Parts_via_Code

It's a long read but it's worth it. Once you learn the basics it will be easier to find out how to do things on your own by just reading the Wiki.