Agreed. Test the battery out. Though usually if the battery is too low to turn the starter, you will get the "death rattle". A clicking noise as the solenoid clicks on and off trying to fire the starter.
If the battery is good, and your starter isn't spinning at all, here are a couple things to try.
Try hitting the starter. As cliche as this sounds when referring to fixing a piece of machinery, the starter when it begins to go bad can lock up prespin. Have someone underneath tap it repeatedly as someone tries to turn the key.
Another thing, you may have a bad solenoid going to the starter. Take a pair of jumper cables and go from positive on the battery, to the post on the starter. This will override the ignition and the relays and should spin the starter right away. If it spins, you have a problem with a solenoid or relay, or even your ignition. If it does not spin (again, assuming you are using a good battery) you almost surely have a bad starter.
If your car will not start, then your starter probably is bad. There are several ways to check and see if your starter is broken but while you look for the problem, do not damage it by continuous cranking because the starter heats up very fats. Read more on http://newcarbuyingguide.com/index.php/news/main/3728/event=view