You are not alone.  Many people for a variety of reasons are not able to do head stand (Sirsasana).  Good thing this is not required! 

 There are many ways to experience being inverted in a yoga class.  Standing forward bend in all of its manifestations can be seen and used as invertions.  Shoulder stand supported or not can be experienced as an invertion.  Being in dolphin pose is an inversion, so is downward facing dog.

 There are health benefits to being inverted for short periods of time.  Yet there are health reasons for not being inverted.  Being inverted increases blood pressure, if your blood pressure is high already, Sirsasana is not for you.  If you have difficulty with pressure inside the eyes, Sirsasana is not for you.  If you experience vertigo, have osteoporosis of the spine, or a neck injury, head stand is not for you. 

 Having said all that, if being in head stand is something you have a burning desire to experience, attend a class where step by step instructions are being given.  Where the instructor can be right next to you, guiding you, and supporting you (perhaps literally with their body) in getting up into head stand.  Know that the brunt of your weight needs to be on your forearms, your interlocked hands and fingers.  Almost no weight at all should be on the top of your head.  You want to be on the parietal portions of your skull, not your forehead.  You do not want to just throw up your legs.  Raise your sit bones up toward the ceiling once you have your forearms in place and your head close to your interlocked fingers.  Walk your feet (you may be on your toes by now) slowly toward your head.  You will feel the balance point when the vertebrae of your trunk are lined up and pulling gently back, away from your legs.  It will feel effortless to take the weight off your toes.  You may now either bend your knees, bringing your legs close to your trunk, pause then lengthen your legs toward the ceiling, or slowly raise your extended legs gracefully toward the ceiling.  Slow is the key here.  Nothing is jerked into place.    

 If the traditional head stand using your forearms as the base does not work for you, you may be part of the percentage of individuals whose humerous (bone of your upper arm) is not long enough to clear the top of your head when the elbow is bent.  If this is you, use your hands to create a tripod with your head and come up into Sirsasana that way.  Even in this tripod head stand, almost no weight should be on the top of your head, the weight should be in your arms, hands and the core muscles of your body. 

 If you are able to do all the prep steps, yet never get your feet off the ground – revel in that final stage of preparation for head stand – use it as your inverted pose.  If you get your knees up on your elbows in tripod head stand and that is as far as you are able to go – celebrate that!

 Being able to do Sirsasana is not the focus of any yoga practice.  If you never do head stand yet have a practice you are dedicated to – you are practicing yoga.  You might be surprised to know that most of the more spectacular yoga poses came along much later in the evolution of yoga and have nothing to do with the original practice created 5,000 years ago.  Some humans simply enjoy doing competitive, spectacular poses with their bodies – it does not mean anyone else has to do them.

 No matter how you decide to be inverted – remember to breathe!  I cannot stress this enough.  Breathe in any difficult pose.  It is your breath that will give you the ability to sustain poses and keep you calm in difficult ones.