First check the battery connections to be certain they are not corroded and are tight. I like to use a lock washer under the wire terminals. It bites into both the battery and the wire and insures a positive electrical connection.
Next check the connections at the block ground and the starter solenoid.
If all these are good, and assuming the engine is turning freely, then you will need to disassemble the starter motor. Check the brushes and commutator, If the brushes are worn or the commutator is dirty--dark colored, not bright copper-- You will need to either replace the brushes or clean the commutator with fine sandpaper--like 200-400 grit.
While you have the starter apart look for burned wires. If there are any burned or broken wires replace the starter.
