Use onshape, it’s so much nicer to use something browser based than desktop based. If you’re a student it’s free.

Watch 2-3 onshape tutorial videos and then just dive in trying to make parts for yourself.

If you’re new to 3d printing you’ll soon learn that what you design doesn’t always print right. Welcome to creating things in the real world. This is part of the learning experience (and why you need to learn by doing).

Onshape also has a bunch of public cad designs, so you can look at how other people made things to learn (sort of like reading source code to learn).