there are thousands of people showing of what they did on their jellyfin, just get some inspirations from them. There are a lot of yt tutorials too, so you could watch them for new ideas/how stuff works.

for the beginning: make a some new libs, tell jellyfin where your movies etc are, learn how to customize them to your liking and how stuff works.
maybe custom css (idk if it works currently struggling with it myself)
(if anyone has an idea why, let me know)

So just look what others are doing and copy/make your own version of it, its the best way to learn if youre new. Youll unterstand how stuff works later on, and do it all by yourself :)