Question for anyone with any CAD experience?

I've been working on a design for an invention of mine and in order to create a prototype I need a CAD model. I would hire someone who knows what they're doing, but I'm a broke college student so that's not really an option. I have no experience whatsoever with any CAD software and I'm not sure if this is even worth my time, but I'm a quick learner and what I'm designing is relatively simple. What software should I use and where should start? I really don't need the most advanced software. Like I said, the design is very simple. Any istance would be greatly appreciated.