Try a new needle, and make sure it is the right size for the fabric...
Also try pulling all the thread out and rethreading it completely.. bobbin, and top thread... Use your manual.. there might be a little part of the threading sequence that you are missing that is causing the machine to not sew correctly..
Use good quality thread.. it can make a difference
try cleaning it and oiling it.. there should be instructions in your manual
its usually the smallest thing that will do this... Sewing machines are actually fairly precise instruments and can be persnickety