Righty Tighty Tension

Every person who works with machines usually has part of the job that gives them fits. For us sewists, that is thread tension. Is it the bobbin? How the machine is threaded? Do I need to twist the knob?

This diagram by Superior Threads is a good visual for deciding what you need to do.
Another guide comes from plumbing and tightening screws: Righty Tighty Lefty Loosey. It even applies to thread tension. The tug of war is a perfect way to think about it. If all your top thread is pooling on the bottom, it means the bobbin is winning and the top needs to pull harder.

If you adjust the tension and are still getting a tangle on one side, try re-threading the machine. Sometimes the thread is not "flossed" into the tension disks. On the bobbin side, make sure that the bobbin thread is coming off the spool in the correct direction. There is usually a diagram in your owners manual. Your machine is also marked with the preferred factory settings. If you have been toying with tension and have all the settings changed, reset everything back to the factory recommendations, re-thread the machine and start adjusting from there. Problems with tension come from improper threading more often than needing to adjust the tension.