Hello Tina,
It sounds like you do not have the tensions set right, first of all make sure you have the presser foot up when you thread the machine, thread from the spool pin to the left and round the top guide that is in line with the tension, down through the tension and round the big guide right and back up the left to the takeup lever, through the takeup lever right to left and follow the guides all the way down to the needle and thread front to back.
Top tension should be about 5, only use a good mercerised thread, try it an see, any problems email me below.
Ken
drivewithken@blueyonder.co.uk, February 2009