Agree with the previous I post.
Google sketchup is great for quick conceptual drawings.
If your looking for more precise construction documents, Autodesk AutoCAD is the way to go. It's a very powerful program that would be more then what you need if you currently just do hand drawings. But the plus is that if you know how to draw by hand, you'll quickly be able to transfer that skill to the computer. I learned this while I was in Architecture School.