Hello Fiona,
Start at the basic, make sure the machine is threaded properly and make sure the presser foot is up when you thread the machine, top tension about 4 or 5, you should be able to pull the bobbin thread through with only slight resistence, if it is too loose tighten the bobbin case screw clockwise about a quarter of a turn at a time until you get it right, make sure the bobbin turns ant-clockwise in the bobbin case.
ken
drivewithken@blueyonder.co.uk, March 2009