Hello sarah,
Chances are it will be the tension, remember that Singer machine work best with Sylko number 50 or 40 and Trylko number 60, top tension should be about 5, make sure you have the presser foot up when you thread the machine and if you have put a new needle in lately make sure it is the correct way round, make sure the bobbin turns anti clockwise and if the thread coming from the bobbin is too tight slacken the screw about a quarter of a turn on the bobbin case, you can get my email address on any answer from page 25 down if you need any help, this site no longer allows any email addresses to be show,
Ken
drivewithken
July 2009