I fell down the stairs and my butt is permanently deformed. What should I do to look like myself again? (photo)

You have scarring contracture in that area, and perhaps some fat atrophy from trauma.   I would wait at least 6 months after the accident, and then consider fat transfer to that area.  You might need a few sessions, and the surgeon should attempt to break up the scarring under the skin.  

Thank you for your question. I fixed several buttock indentations with liposuction of the surrounding area, release of scar bands, and fat transfer. The results are excellent and permanent. Best wishes to you.

Dear Practical3153,
it is hard to tell for sure without an examination. Based on your photos and description it sounds like you would benefit from fat transfer. If you are considering a surgery, I would suggest you to consult a board-certified plastic surgeon. Only after a thorough examination you will get more information and recommendations.
Daniel Barrett, MD

Hello, thanks for the question & photos. Sorry about your accident. You are a candidate for a fat transfer to your buttocks. You'll need a little amount to fill the dented area to reach a symmetrical result. Since you are young and athletic, you can have excellent results. Find a board-certified plastic surgeon in your area for an in-person exam. Good Luck!

Do not feel so bad because you are not the first or only person to have this happen to them. I have had multiple patients with the exact same scenario. This can be corrected with a release of the scar through a very small incision and simultaneous fat grafting with excellent results

Hello dear!
Thanks for the question and provided information as well.
I'm sorry for what you went through. Based on your pictures, you are a good candidate for liposuction with fat transfer, to improve your buttocks and fill the dent, and I think you'll get very good results. 
In general, you need to be physically healthy and at a stable weight, have realistic expectations, be a non-smoker and your BMI needs to be 30 or less.
l recommend you to make an appointment with a board certified plastic surgeon to talk about your goals and anything you need to know.
Good luck :)