Google Sketchup is good for beginners, also downloading the soapskin bubbles plugin is a good idea. Sketchup can get a bit buggy and annoying, but it is the easiest to use.
I personally like to use Autodesk Inventor for more complex projects or if I'm bored. You have to pay for it unless you can get a students license. But it is worth it.
Also, if you want more of an artistic view, go with Blender. Blender is a little more complex than the two above, but it is still neat to work with lighting and realistic renderings. Blender is free, but again, is the hardest to pick up.