Wont start part 2 ( Please Help Desperate)

well here's the problem i have a 1993 mazda 626. On saturaday i drove the car twice and then 3 hours later when i went to start the car i got nothing. When i turn the key i get one intial click followed by a light humming sound. I know the problem is not the battery because i've tried boosting it with no effect. Personally i think the problem is the starter or the solenoid but im not sure which one it is. My dad says the problem is probably a relay but im not sure which relay would be causing the problem. Please let me know if you think you know what the problem is i really really need this car.