when i turn the key to start,it just clicks rapidly and does not turn over. on 2001 Ford Expedition

i have tried using a battery booster to start it and it does not click or turn sounds like it is coming from the firewall.I thought maybe it was the starter but if i am not mistaken it is under the car.this is the first time i had this problem.
thanks wayne

Asked by for the 2001 Ford Expedition
Clean the battery cables and the post! Now clean them again because you didn'd do it right the first time! Tighten cable clamps good. Now try to crank without jumper box. Post results!
while i was waiting for your response i went and put my charger on it for 1/2 hour and then i cleaned the posts ,but were not that bad,and then i used my booster and it started one turn.thanks