There are 2 cables to most starter motors. A heavy cable that comes from the + battery terminal to a terminal on the starter solenoid, and a second thinner wire that comes from your ignition switch, usually via a relay. That cable lug needs to be tightly secure to the solenoid terminal post.
The earth cable is usually secured from the battery to a point on the engine, usually on one of the bolts that join the engine and gearbox together. It could also be bolted to the starter housing using one of the bolts that holds the starter motor.
A steel bolt and an alloy thread carrying a heavy current can cause corrosion to the alloy over time which doesn't make a good contact. If it isn't secured tightly then you must do that.
With regard to your earth cable, clean all contacts and protect with copper grease. You can also copper grease onto the bolt threads so they will also be protected, but you must be careful not to over grease.
If you have a set of jumper cables, take the black one only and put one of the spring clamps on the negative battery post and put the other on a bolt somewhere on the engine and see it it starts regularily then.