Getting to know your sewing machine tensions.   The bottom tensions of your sewing machine very rarely needs adjustment so we will concentrate on the top tensions.   The top tension is only metal plates pushed together by an adjustment knob and spring to create a measured drag.   The normal setting is about 4.   Now, why don't you just sew  a little on a piece of fabric and then look at your tensions.   Now turn your top tension knob to the right about 3 numbers (higher numbers), then look at your stitches.  You should see your upper thread being drawn to the top of the fabric.    Now, go back to your normal setting (4) and turn your top tension knob to the left about 3 numbers (smaller numbers), then look at your stitches.  You should see upper thread being drawn to the bottom of the fabric.   See you didn't break your machine and it didn't even bite.
But, what to do if I'm having problems.   Well, the very first thing to do is clean and, if needed, oil your machine.   Just like any tool the better the working condition, the better the results.   And, it is always recommended for you to do a little service before you start a project.   The second thing to do is change your needle and making sure you have the right needle for the project.   A dull, bent, or defective needle can cause what appears to be tension problems.  Also, a damaged or improper needle can cause skipping and fraying problems.   The third thing is  to re-thread your machine using good thread.   Bad thread can also cause tensions and skipping problems.   Making sure your presser foot lifter is up -that will make sure that your tension plates are loose so that the thread can be easily pulled in between into the plates.   Make sure that the thread is pulled into the bobbin case tension.  While you are threading your machine, feel the thread.    It should be smooth and flexible.
Now let's test sew on some fabric.   Making sure your top tension is set on normal (4), you are on straight stitch with your length on 2-1/2 (12 stitches per inch), sew about 8 inches.  Now look at your  stitch.   Remember what we did when your were getting to know your tension.   Adjust your tensions so that they look the same on both sides.  You should only adjust your top tension 1 to 1-1/2 numbers either way to get a good balance.   If your stitch still looks unbalanced, check and insure that your thread is pulled into the bobbin case tension spring.

Your Serger Tensions  are similar to your sewing machine.   And, like your sewing machine clean and oil your serger.  Remember to change your needles, this is much more important than on your sewing machine.  One difference is that most serger tensions are not released when the presser foot is lifted.   So make sure that you pull your thread into the tension by holding the thread before and after the tension assembly.   When adjusting your tensions always look at your left needle thread first.  That is your base for the rest of the stitch.  Set your tensions to normal ( usually about 4 ) and serge about 6 to 8 inches.   Look at your left needle thread.   It should look balanced similar to your sewing machine stitch.  Next, look at your looper threads.  They should be joining on the center of the edge  of the fabric.   If the stitch is going more to the top, loosen your upper looper tension.  If the stitch is going more to the bottom,  loosen  your lower looper tension.  Now, if you have a 4-thread serger look at your right needle stitch.   If it is coming more to the top, loosen the tension for your right needle.   If it is going more to the bottom, tighten the tension for your right needle.   Two things to remember: You should always try and keep your tensions on your serger as loose as possible.   This will cut down on thread breakage.  And, always use good thread.   The serger stitch is being formed by all the 2, 3, 4, or 5 threads being used.   If one of those threads is not good and consistent, your could have a tension nightmare.   
If, you are still having a problem after all of this,  check our Thread and Needle pages for more help.   And if all else fails,  call your local dealer for help.  Keeping in mind that there is probably something more than a tension problem.  You might even have a damaged part.   So, you might just consider having a service done.   In fact, it is a good idea to have your machine (sewing machine and/or serger ) serviced every 3 to 5 years.