You said older machine, so I'm assuming it's all manual. Put different colored threads in the bobbin and on top. Sew a three or four inch line of stitches on some kind of fabric you usually use and the number of layers that are your usual (or whatever is giving you problem) and look at it. If your bottom thread is showing on the top, you need to loosen the top tension. If the top thread is showing on the bottom, you need to tighten the top tension. Rarely, you may need to adjust the bobbin tension. Generally speaking, you need a tiny screwdriver and the instructions from your manual to do that (make tiny, tiny adjustments on the bobbin and test, then adjust again and test). There is generally a screw that gets tightened or loosened. It's not hard to do. Optimally, you want the the tensions to be perfectly balanced so that you see the same amount of thread on each side. Usually, you can do it by adjusting the top tension only, though.
It wouldn't hurt to clean the tension disks before you start the process. You just take something like dental floss, pipe cleaners or something and run it between the disks.
There are some great tutorials on here on adjusting tension. Do a search and you'll find many. Refresh the page after you open it, or it will appear blank.
If you don't have a manual for your machine, you can download one of the free ones at ISMACS.net. The process will be slightly different if you have a drop in bobbin or a removable bobbin case.
I have to change the tension on my vintage machine when I use a walking foot and with different threads. I find that I really have to loosen the top tension when I use clear thread.
Oh, and the bottom is just a straight line of thread. The top thread loops around the bottom thread to give you a stitch.
bkay