How should I position a subwoofer that is part of a 2.1 computer speaker setup?
Is it supposed to actually make music, or just a bass sound?
No, a subwoofer typically only emits frequencies well below 300 Hz, which we don't really hear as notes or actual music, but rather just a "thump" or … well … moving air. A subwoofer is built for this, with its larger membrane being able to reproduce these frequencies better (and without distortion).
It's supposed to be that way, because the smaller speakers take care of the rest of the (higher) frequencies. Your subwoofer normally has a built-in frequency switch that splits up the signal. Some (more expensive) subwoofers also allow you to define this "crossover" point.
Is it normal that I have to set up a very high sound level for the subwoofer?
Whatever feels or sounds right for you is fine. There's no way to tell, as every subwoofer is different, and your room typically plays a large role when it comes to how your system sounds. Just experiment and see what sounds best for you.
Is it ok for it to be under the height of my head?
The human auditory system does a better job at locating the source of higher frequencies than the source of lower frequencies. Because of this, it is not that important where you place your subwoofer (i.e. to the left or to the right). For the smaller "satellite" speakers, minor position changes could make a difference though.
As per Wikipedia:
[…] so a precise evaluation of the input direction is nearly impossible on the basis of level differences alone. As the frequency drops below 80 Hz it becomes difficult or impossible to use either time difference or level difference to determine a sound's lateral source, because the phase difference between the ears becomes too small for a directional evaluation.
Where do you place it then?
Some people prefer to put it on the floor to reduce shaking or vibrations, whereas others like to "isolate" it from the floor. If the subwoofer interferes with your room (by being too tightly coupled to the floor or other objects), it might start sounding bad. You can try placing it on a cushion or a piece of styrofoam.
You will want to put your subwoofer out of your line of sight though (so under the table should be fine). The reason is that you would otherwise pick up noise that's not intended for you to be heard. If you listen to the subwoofer a) on your desk and b) on the floor, you will hear what I mean.
Again, try different placements and see what delivers the best sound for your ears.