If your stitches do not look good on the top side you have a problem with the bottom thread. make sure you have the correct bottom tension: Is the thread under the bobbin tension plate and when you hold the bobbin and case lightly in your hand the tension should be set in a way that there is little resistance , yet still some, when you pull the thread. After you set your bobbin tension, you may have to adjust your top tension to balance. Also make sure the thread comes off of your bobbin the way your manual tells you, clock or counter clock wise.
Make sure your bobbin (plastic) is not sooooo old, buy a new one.