This handbook will introduce a few of the most popular 3D modeling software options currently available.
Quick, non-complex modeling
Tinkercad is pretty darn good for most things. The ruler is a bit tricky if you're trying to do precision, though.
SketchUp is also very friendly and good for architectural things.
Inventor, AutoCAD, or the pretty popular web-based (and free) Onshape.
Blender is the best. 123D Sculpt+ on iPad is also pretty good though, and way easier to use.