Tension is a common problem for new and even intermediate sewists. Testing is about the best way to figure out what works for your machine.
Be sure you are using a brand new needle and that it is installed correctly (flat side facing the proper direction).
ALWAYS RAISE the presser foot when threading the upper thread. This releases the tension disk so the thread will seat completely. Check the thread path and verify that it is correct. Missing one thread guide will create havoc!
Check that the bobbin thread is wound smoothly and that there are no thread ends sticking out of the bobbin anywhere. The bobbin must turn in a specific direction when in the bobbin case and the bobbin tension needs to be threaded correctly. (For this, you need to consult your owner's manual.)
Set the upper tension to the mid-way point. This is the default setting although it will most likely require some tweaking because all machines are just a bit different. Tweaking is a very common necessity as different threads and fabrics will affect the tension.
Set the straight stitch length to somewhere around 10 stitches per inch.
Use two pieces of medium weight fabric, like 100% cotton. Bring the bobbin thread to the top of the fabric before beginning to stitch.
http://ohyoucraftygal.blogspot.com/2013/10/sewing-lesson-10-how-to-fix-tension-on.html Understanding Thread Tension Threads
Test your stitch. The ideal tension is when both the top thread and bobbin thread meet in the middle of the fabric.
If the top thread is looping or showing under the fabric and the bobbin thread appears to be running in a straight line, the top tension is too loose.
If the bobbin thread is showing on the top of the fabric or the upper thread is running in a straight line, the upper tension is too tight.
Keep adjusting until it looks like the pictures in the web links above. Remember, when you change thread or fabric weights/types, etc., you will probably need to re-test the tension and adjust.
One more suggestion: use good quality thread. AVOID old or bargain bin threads. They may save you money but will cost a lot in frustration.