Do you need anything specific like an app on the computer to make games for this site? My cousin just joined this site and he likes to make games maybe not this type of game but just in general?
Uhm, off topic but here are some pointers for you: Itch.io is just a marketplace for games. You can make a game using any kind of game development software and offer it for download here.
If you're talking about making visual novels, which are actually a good place to start for a dip into game development, RenPy is a very popular engine for creating visual novels and it's easy to get into.
Though you have to keep your expectations in check, you can't create a game like this by just using RenPy. But let's not take too many steps at a time. Best of luck to your cousin!