After you have read the manual, rethreaded the machine and replaced the bobbin, be sure the hook is in the up position before you sew. Hold the threads in your left hand, drop the foot and start sewing slowly. After a few stitches, release the threads. It is important to start any seam with the thread hook on top, where it is when you can easily pull the threads. That is the first thing I tell any new sewer. If you don't you will have a snarl of threads and a problem getting the needle up and a mess to clean up. This happens to the best of us.
Carol J.