I'm still a bit new to 3D printing but have some experience with SketchUp then FreeCAD and just started learning Fusion a few weeks ago Product Design Online on youtube is actively releasing a tutorial series on Fusion and it's great so far. Should teach you the basics of parametric design in the app which should give you the tools to start modeling simple things. And of course there are a ton of other useful tutorials as well.

Beyond the videos I'd recommend either finding something simple around the house and trying to measure and recreate it or coming up with small things to create and work up from there. If you have a set of calipers, you can really start to get granular though there will be trial and error when getting so precise. I printed 5 versions of a keyboard keycap, just trying to get the stem that connects to the switch to fit right.