To be honest, you got your work cut out here. A decent inventory system is a WHOLE THING. And yo have two pretty detailed parts. The backend, which usually involves a lot of structures and using the save game, and the front end, which is a fairly complex widget system.
A good place to start is by getting this:
It’s free, and has a good inventory system built in ( so you can see how that works ). I can’t recall what kind of interface there is but…