When l turn the key in my tracer the engine makes a reving sound like it is just about to start but it wo not turn over. Also, there is a humming noise coming from a little box in the engine (constant control relay box?)

take it to a car dealer ship/garage businessman type store.

Sounds like Starter or battery Clean cables on battery and try again if no try jumping it off if it starts your battery is going bad hope this helps.