Assume you are attempting to start with the clutch depressed and that ambient temperature is 40F or better.
If lower ambient temp, ensure Thermostart is functioning.....Many posts herein on what it is, where it's located physically, and how it works. It takes 30 seconds to start slobbering so holding your key in the TS position.....where the red lights go out between ON and Start positions for like a minute is best for me.
10V at the starter case (-) and 3x8" copper stud (+) where the battery cable attaches to the starter...stud proper, not a cable lead or something else........while rolling the engine.
If OK, compression low, or faulty starter. If it starts ok after warmed up, then forget the faulty starter part.
If not, Follow the voltage. Anything that isn't up to par will have a voltage drop across it or will get warm as you crank.
If the voltage across the actual battery terminals (not the cable connectors) isn't 11v while cranking, have the battery load tested to 300 amps. If it can't hold ll volts at 300 amps replace it. If it can, recharge it to full capacity and go after your cabling ensuring that all cable connections have been removed, cleaned till shiny and reinstalled with the proper torque.
Not a difficult process. Just follow the voltage.