Before starting, remember that the bobbin tension must always be less than the top thread tension.
Some problems and their solutions.
Problem: puckering
Cause: Assuming here that you have already adjusted your presser foot. If your thread tension is too tight, the thread will pucker the fabric as you sew. Reason is that the thread pulls the seam together. It looks terrible and makes your work look cheap (look at the seams on a cheap shirt that has been worn a few times).
Solution: Loosen the top thread tension.
Problem: Snapping thread
Cause: see above, or maybe you're using vintage cotton thread.
Solution: If the latter, just know that old cotton is generally not usable for sewing. Throw it out or use it for a display.
Problem: thread loose and loopy at the bottom
Cause: Top tension is too loose
Solution: Increase top thread tension
Problem: Thread loose and loopy at the top
Cause: Bobbin tension is too loose
Solution: Increase bobbin thread tension.
More likely you'll have something in between and your stitches aren't balanced. Your stitches should be perfectly balanced, with a dot from the bottom just visible between top stitches and a dot of the top thread just visible between stitches at the bottom. Test this using contrasting coloured threads.
Top tension
Almost all tension problems can be fixed by adjusting or fixing the top thread. The sewing machine was designed like this so you don't have to fiddle about with the bobbin, which is always more difficult than adjusting the top.
Thread your needle, making sure the thread is between the tension discs.
Make sure you thread it with the foot up. Reason is that when the foot is down, the discs are pressing together and will not allow the thread to sit between them. Then there is absolutely no tension on the discs. If there's no tension, chances are you threaded it while the foot was down.
Now put the foot down and pull lightly on the thread, through the needle. Not hard, and the thread should deflect (bend) the needle. Now turn the top tension toward zero. At around 1 the thread will start moving and at zero there should be almost no resistance at all. If this is not the case, you will need to adjust, or calibrate, your tension dial. If you don't want to, just don't rely on the numbers. From zero, most machines are happy for normal clothing fabric at around 2.5 to 3.5, at least on my old Singers.
I'm not going to tell you how to calibrate your machine's tension dial because your machine will almost certainly not be like mine.
Bobbin tension
If you absolutely must do this, how you do it will depend on whether it has a vertical bobbin (with removable bobbin case) or horizontal (drop-in bobbin - non-removable bobbin case).
Vertical bobbin machines: Remove the case and holding the end of the bobbin thread, let the rest (bobbin and case) hang. It should only just be able to stay still so that if you move the end of the thread up at all the thread will unravel. If this is not the case, adjust by turning the little screw in to tighten or out to loosen. Keep testing until it is perfectly balanced. Adjuster is on the tension spring, which the thread passes through.
Horizontal bobbin: Thread it through the throat plate like you were preparing to sew. Pull the end and adjust as neccesary so you just feel a little tension on the thread. Adjuster is on the tension spring.
Before I learned about all of this I probably shook with fear if a machine's tension wasn't right, but it's really not that bad. If it loops, the side opposite to the loop is too loose. If puckering, loosen the top off. For puckering, check the foot pressure before adjusting the tension or you'll have to fix the tension afterwards.
