I'll agree that Autocad, Autocad Inventer are excellent choice but $$$ a lot..
I've been using Sketchup and very easy to use..
I've been building my future tank through this program!
pro-e wildfire, catia, solid works ,auto cad....i do all my drawing on the clock at work lol..mostly with autocad/pro-e.....google sketchup is an ok simple program...i would use graph paper and pecil first but each to there own