Tension is one of the biggest problems for sewers and quilters. Well, if you've got them they're BIG. If you don't, consider yourself lucky!!
The point where the upper thread and lower thread meet is critical in the tension issue. That intersection should be right in the middle of your 2 (or more) layers. You should not be able to see the top thread on the bottom or the bottom thread on the top. If you can see the top thread on the bottom fabric, it will look like pokies coming through the holes made with your needle and your b0ttom thread will just be laying on the fabric held in place by those pokies. In the following pictures the pink thread is the top thread and the brown is the bobbin/bottom thread.
If you have problems, start with adjusting the top tension. The larger the number the tighter the tension (or check your manual). So if you can see the top thread on the bottom, tighten the upper tension a touch. Test stitch again. Tighten as needed. If you have the top as tight as it will go and it's not better, then you might have to adjust the bottom tension. On machines that the bobbin case comes out, there is usually a little screw on the bobbin case. Use a tiny screw driver (usually included with your machine) and tighten or loosen the tension on the bobbin thread. Remember: Right is Tight and Left is Loose. So in the situation above you want to loosen your bobbin tension just a touch. Run a seam of test stitching. It may take a few back and forth adjustments and it's best to just move the screw a hair each time.
For those of you that have the drop in bobbin assembly, I have no clue how to adjust your bobbin tension. I'm sure there has to be a way so read your manual and check it out.
If you can see the bobbin thread on the top, loosen the top tension and/or tighten the bobbin tension. If you can see the top thread on the bottom, tighten the top tension and/or loosen the bobbin thread. You can do it!! Don't be afraid, and get to know your machine.