Hello Pauline,
First off it sounds like a top tension problem, make sure first of all that the presser foot is up when you thread the machine, top tension between 4 and 6, bottom tension enough to be able to pull the thread through with some resistence, try that first, if the thread is still getting tangles up underneath after checking the threading and tensions then look for sharp bits onj the bobbin case that may be dragging the thread down, if everything is smooth including the hole in the needle then you may need to retime the machine, I can attach timing instructions if you e-mail me and remind me of your make and problem.
Ken
drivewithken@blueyonder.co.uk, April 2008