Why is my Mazda overheating and not starting? Please help. :(

My Mazda overheated one day, so then I shut it off. When I went to restart it, it wouldn't restart. My friend replaced the altenator and it was fine. About a week later, I was driving and it ovcerheated again. Once again, it's not restarting. Im guessing it is the altenator again. The problems I've noticed is that it does make a squealing sound, there feels like there is heat coming through the vents when the heat isn't on. I also noticed a leak under the car, and was told it needs avalve cover gasket and plug wires.