You might want to check out Articy:Draft by nevigo if you are in the process of designing skill tree and party content. It’s great for that.
Hopefully, given the exposed source code and friendly devs, someone will make an API for articy to UE4 in time.
Anyway, within UE4, before you make those things you have to learn about
- Saving var values as data on disk (or you’ll only be able to level up within one game session)
- UI, especially text and text inputs (like user enters character name)
- Comparing the input string to see if it’s valid and otherwise matching it during run-time
- Conditions for figuring out which exp. level is current or next or which skill
- States and switches for changing anything relevant based on what level is current