Hello Noor,
Either you do not have the presser foot up when tghreading the machine this will stop the thread going between the tensions discs or it is a tension adjustment, makew sure the top tension is about 4 or 5
pull the thread through on the bobbin so you feel slight resistence, if it feels too tight slacken the screw on the bobbin case anti clockwise about a quarter of a turn at a time until it feels right, make sure you have the bobbin in the correct way round so it turns anti clockwise.
Ken
kendourof, November 2010