For reasons lost in the mists of time, the "relay" is, in the context of starting circuits, called a "Solenoid"
A solenoid is a switch for high current circuits (thick wires, chunky terminals) that is controlled by a much lower current applied to a pair of separate m, light weight terminals with relatively thin wires.
Without knowing the exact details of what you have in front of you, the following is a generalisation. You'll need to think things through and/or do some searching of your own
There will be a thick feed cable, coming probably directly from the battery positive. This will go to one of the big terminals on the solenoid.
There will probably be a similar thick cable going to the starter motor. This feeds the heavy current to the starter motor and will connect to the other heavy terminal in the solenoid.
There will be a thinner cable coming from the ignition switch starter contact. This feeds the low current part of the solenoid to operate the heavy contacts. This will go to one of the lightweight terminals.
The remaining thin cable will probably go to the other light weight terminal.
Ensure the thick cables connecting the engine and the battery negative to the frame/chassis are in good condition and connections are tight.
A simple Duck duckgovsearch will throw up lots of diagrams for starter solenoid circuits. They will all be subtly different but it should be easy to trace the light current and heavy current paths.