For this kind of project, SketchUp is a great choice. MUCH simpler to use and faster to learn than a "regular" CAD program. There's a free version available from Google that you can try out.-Jimdoria
One caveat with SketchUp: I've found it difficult to do exact dimensioning. It requires some mouse-and-keyboard-fu, and changing dimensions after the fact is difficult enough that I've found it easier to delete missized objects and start over. I'd prefer a parametric CAD system for generating plans for fine furnishings; that is to say, anything I was putting inside as opposed to a garage.
Or, you can use pencil and paper, as I did for my headboard, which came out fine. I'd suggest getting a good, ghosted grid paper; I'm partial to the
engineer's computation pad myself. It is green paper with the grid printed on the reverse so it just shows through. Of course, I'm an engineering student, and always have this stuff around, so I may be biased. A light, ghosted grid is essential, though, since it lets you use an erasable pencil without (1) damaging the grid lines, and (2) confusing your lines with the grid.