You can begin practicing the head stand using simpler inverted poses. These help you get used to the feeling of being upside down and if you can't do the headstand to start, you can use these to practice and develop your ability. The Dolphin
poses are just the beginning steps of the head stand: rest your head on the floor with your elbows down, as noded above, and straighten your legs to form a triangle (you sort of look like a dolphin or pyramid) then slowly move your toes closer to your head, flex the toes and hold there.
One can also enter various advanced head stands to develop specific skills once you have mastered the first. They all start with the basic headstand noded above.
The second headstand is attained by spreading your legs and flexing your toes towards the ground. You will probably lose your balance the first few times you try, so keep praticing.
The third headstand has your lower (or upper, if you're upside down) body twisted. In the first head stand, simply twist your body from waist down(or up) to the right as far as you can, holding, and then to the left as far as you can.
To tighten your ass cheeks, you can put your feet together and bring them as close to your crotch as you can while in the head stand.
The most advanced head stand is the head stand in lotus. First practice going into the lotus on the floor; then try it while in the head stand. It is best if you can go into the lotus without using your hands, because you need those to balance! To practice, you can start by going into the half lotus.
There is a simple pattern in these advanced poses that is recurrent in many of the inverted poses of Hatha yoga, like the Shoulder stand sequence.