Depends on what you want to do in the software, I use Photoshop in my day job so I use that to make mock-ups to see how a design will look but they aren't to scale just pretty pictures, there's a free program called GIMP that is similar to Photoshop that you could try, it would be ideal for sketching out ideas.
AutoCad is the big cheese when it comes to technical drawings but it's expensive and complicated, maybe someone else knows of a cheap alternative?