Make an appointment with your vet soon. Make sure to take your food/treat lables and any shampoos you use with you to your vet, as he/she may ask about those things then you'll already have them.

Hair loss can be caused by a large number of things; ringworm, fleas, staph infection, mites, allergies, thyroid problems, mange, etc.

Only your vet will be able to tell with an examination (and possibly: skin scrapings, looking at your dog with a special light, blood work) what is causing the hair loss on your particular dog.

I wouldn't wait though, skin problems tend to get kinda nasty if you don't get them taken care of right away, and some of them can be contagious to humans. Call your vet for an appointment.