Honestly, just playing around with it has been what's worked best for me. Go to printables and find some cool stuff and then read their comments on recommended print settings. When you run into problems (prints not sticking to the bed, warping or whatever), look for the answers with your search engine of choice. Once you can print out models that are pre-made, next you can get into CAD if you're so inclined. There are tons of good videos on FreCAD on yewtu.be.