What thread brand/weight/content and needle size & type are you using?
My first guess would be the upper tension is way too high, but start with the basics. Unthread completely and clean. Pay special attention to the tension discs, flossing them to make sure there's no thread or lint between them. Even if you have had your machine for a while, get the manual out & rethread using the instructions. Wind a new bobbin, and insert a new needle, going up one size.
If you are still getting breakage, try lowering the upper tension. Also, make sure that you are not pushing or pulling the fabric as you sew. Let the feed dogs do the work.