Make sure you can turn the tension nob from about 0 to 9, (maybe even 0 to 0). When pulling the thread back behind the presser foot it should fee loose and gradually firmer towards 9. Double check the thread, no loops under the spool pins or catching along the way.
Sewing two layers of cotton (shirt or quilt weight) with sew-all weight thread, it should give a balanced stitch some where between 4 and 5.