Do you have any mechanical knowledge and experience?
A battery can show good voltage levels and still be bad.
Bring the battery to most any auto parts store and have it load tested. If it fails, replace it.
Check to make sure connections to the battery are clean and tight. Same with the starter solenoid and starter.
Does the starter/engine turn over? If not, jump the starter solenoid. Starter/engine turn over? If not, the problem is between the battery and starter including the starter and grounding system.
Else you'll need to check the ignition system, and that get's involved.