if its the bottom thread that is lose and its the standard bobbin there is a small screw on the bobbin casing adjust by tightining or slacking very slightly if its the Top tension then adjust the tension on the front of the machine hope this helps you could alway look up instruction book on the net for your machine and this might help hope you get there anyway good luch

check the bobbin tension - there usually is a + / - sign on the metal holder that holds the bottom thread bobbin and also the stitch tension wheel depending on your make/model I usually have mine between the 4/5 mark. Hope this works for you!