+1 for learning SketchUp. There's so much you can do with the free version.
TheSketchUpEssentials is my favorite YouTube channel for learning new techniques. He is a very good instructor.
https://www.youtube.com/thesketchupessentials
If you learn some basic keyboard shortcuts (O to orbit, R for rectangle tool, P for push/pull, etc) you will be building scale designs in no time. Being being able to orbit a design in 3D and visualize it from any angle before any cutting and assembling is incredibly useful.
This playlist might be a good place to start and get oriented with it.
https://www.youtube.com/watch?v=vFVwPnDxNY4&list=PLEQT0qjXe6zgno3Hz0F_nWjEgL7KxsJnI