When I took a class for machine quilting, we used the book by Maureen Noble "Machine Quilting Made Easy" and she had a great tension exercise.
Make a 12-14" sandwich of plain muslin with whatever batting you are going to use. Mark the square with horizontal lines ~ 1" apart and number each line starting from 0.
Thread your machine and set your tension at 0. Sew across the lines from marked 0 to marked 1. Change the tension to 1 and sew to marked line 2 (and so on).
When you remove your sandwich you can REALLY tell which tension works best for that particular setup with your machine.
It is suggested to do this exercise when you change thread weights or some other factor.
I felt much more confident in trying new things once I figured out my machine.