Take a look at
www.sketchup.com
Very easy to use 3D software and there is a free version - just won't let you output to dwg or plot in higher resolutions. Personally, for something like a garage or shop it really helps to have 3D to figure out how things are going to fit. And you can download "models" from Google of cars, tools, workbenches, shop equipment, etc. I've had the pro version for about four years and love using it.