First check to see that your threads are properly threaded accordingly, before serging grab the back threads firmly and make a chain at least 2-4 inches long, stop, lift your presser foot, insert your fabric and serge away. When you finish your desired serging, chain off but keep it serging to leave a short tail behind the project, or else the serger will jam up or break your threads again. Don't forget to lubricate your blade accordingly. If you serge lots of items, the blade will require oiling.
Hope this helps!