I've had good luck with all of these: Sketchup, Fusion 360, Rhino, Alibre
All have (had?) free options. Sketchup may be the easiest to get going with. Fusion includes some integrated CAM tools if you're going to do some fabrication.
I recommend a parametric modeller (Fusion, Alibre) if you're going to be playing with fitting parts together. This lets you create your model(s) with parameters, such as, "This box is 2 mm longer than the part inside." Then, as you change the part inside, the box will magically change to match.
Without parameters, every time you change one part, you have to go manually change the other part to match, which becomes a HUGE pain as your models get more complex.