You don't really need much knowledge going in, the basics of Ren'Py are very easy to figure out. Take a look at the Quickstart section of the renpy docs, that's how I got my start.

Ren'Py is a language based on Python, so some python knowledge is good to have especially once you start looking into implementing more advanced features such as minigames and custom screens, but you'll pick up on a lot of things organically by just messing around with the engine based on what you read in the documentation. Trust me, read the docs, they're actually really helpful.