Hello Susannah,
First of all check to make sure you are putting the thread between the tension discs and not to the side of it, very easy mistake to make, make sure the presser foot is up so the thread goes completetly between the discs, top tension between 4 and 6, be sure you have threaded the machine right, bottom tension enough to be able to pull the thread through evenly and not too tight, look for any damage on the needle plate.
drivewithken@blueyonder.co.uk, January 2008