I don't believe you said whether or not the bobbin thread is coming to the top. If it is, I'd go with the suggestions for readjusting the tensions (either loosening the top or tightening the bottom.) Try changing just one at a time and use 2 contrasting threads that are a different color from your test fabric. It's way easier to see it that way. And definitely a) draw a picture of the bobbin with the starting position of the screw on it and b) record the starting tension of the upper thread. That way, you can return to your starting position. This will make you braver in doing it, too.
hugs,
charlotte