Battery or Starter?
It's going to be either the battery or the starter. The voltage you are reading is a bit low. I'd have the battery load tested at your local Autozone or Advanced Auto parts.
A starter is a high torque motor. It has to overcome the compression of your cylinders. If you can get it to turn over with the plugs out, but not with them in and your battery is good, your starter is on its last leg.
If you have jumper cables or a jump box, you can try to jump the battery before you do anything. If you use a 12 volt battery from a vehicle, make sure the jump vehicles motor is shut down. Do not try to jump start from a running vehicle, damage to the electrial system could occur.