Hi and welcome to the site,
Initially, have the battery load tested.
Offhand, temporarily jumper the 2 big terminals of the starter relay. If the starter motor spins, then a faulty starter relay. To locate the starter relay, follow the cable from the battery's + terminal.
If even jumping the 2 terminals, the starter motor would not turn, either:
- starter motor has issues more particularly the carbon brushes and/or the commutator segments; or
- the battery is no longer able to hold sufficient charge.
Good luck and thank you for asking.