When I purchased my 3D printer almost a year ago, my main interest was in being able to design my own objects to print. I was able to print some simple mounts using SketchUp but have struggled to advance beyond simple designs for a long time. Recently, I learned about OnShape from my local makerspace. OnShape is a browser-based, hosted CAD system and is free to use if you make your designs public (you can have 10 private documents too).
I've been learning how to use OnShape for around a month now, and have finally been able to come up with some relatively complex designs. I'm currently designing all the parts for a Dalek as this is a good way to force me to learn many features, given the different geometries involved. Here is the dome and eye stalk, which are modeled as separate parts, but one of the nice features of OnShape is that you can then put these parts together to make sure they fit.
OnShape has some really great video tutorials that have helped me get up to speed. I expect I'll be learning a lot more over the coming months. My end goal is to be able to make some mechanical parts and start creating some fun robots / animatronics.