I would check the battery, and the battery ground going to the engine by the starter first. If it looks good and clean, Check the starter solenoid. Follow the Red + from the battery to the sealed electrical box. Open it up and using a 14 guage insulated wire jump across the solenoid. If it rotates the starter it is more than likely the solenoid. If the starter doesn't rotate it's the starter.
I hope this helps.
Karl