Brenda10, what kind of symptoms are your getting? What kind of machine is it? Do you have a manual for it to help you troubleshoot problems, thread it correctly, etc? Does it act up every time you try to sew, or only when you are doing a certain stitch or only when you are trying to sew specific fabrics?
I know there are lots of people who will help you on this board, but we need a little bit more info to go on. :)
The general instruction is to set the bobbin tension for the thread you're using in the bobbin.
Use the right needle in the right size for the thread that you're using on top and thread the machine correctly.
Balance the needle tension with the bobbin tension so that the two threads are interlocking between the fabrics. You should have a line of even stitches that lie smoothly on each side and no puckering of the fabric. :)
If you aren't getting this, then describing what the stitches look like will help people figure out what's wrong.