Sketchup is an interesting CAD-type (Computer Aided Design) drawing/modeling tool from Google, available as a free and paid version. The software has been around for a few years now, and you can use it to produce designs from scratch, or import items made by others and adjust to your needs.
Sketchup is not the sort of program you would use to make animation or organic shapes. It is basically a CAD program specializing in rigid body objects: buildings, tools, device cases and the like.
The operation of the program is pretty straightforward if you are at all familiar with any kind of 3D modeling software, but may not be so obvious to the rest of us. The last time I used Sketchup, I found it a little too user friendly, in that it tried to anticipate what I was doing a little too much for my liking.
That was several years ago though, so the newer versions may not share that trait. In any case, if you want a quick primer to get you started using this free software, MakeTechEasier has just the thing for you here. This is the first in a proposed series of articles of Sketchup at MakeTechEasier.
The are also some video tutorials here at the Sketchup site, if you prefer that type of help.