Someone Help Me with My Doggie Problem

Ok, my pup is about five months old, and she is a Black Lab/ German Shep Mix…

She has turned aggresive… im not sure what is going on, but im trying to train her and she doesnt listen…. i let her out, and she darts off… i put her on the lease and she attacks me and the lease… when i say attack, i mean she is literally trying to bite my freaking hand off… i have had her since she was six weeks old, and i really dont know what the crap is going on…

Im trying to get her into training, but i really dont have alot of money for that.. when i feed her, she gobbles it all up…and when it take her out, she doesnt want to go then trys to bite me… Im trying to assert myself as the alpha…but i really dont know what else to do…

What am i doing wrong… someone please give me some ideas, suggestions…anything…