Why my car does not start and dies out?

I have a Mazda protoge 5 that's giving me problems. It all began after I had my serpentine belt changed. It did good, the first week but, Since then my car has been squealing when I start it and now it has been dying out on me. I thought it was my battery so I went and purchased a new one. The radio, lights, and everything in my car still turns on but it does not start up when I attempt to start the car. Does anyone have any suggestions on how to fix the problem or know any web sites that can help with the problem?