* wall sits 4-5 sets , reps would be to failure. Til you can't take it anymore to sit at a 90 degree angle against a wall.
* recommend barbell squats with even lighter weights.
- with 5-10lbs in either hand held hanging at your waist go ahead and do squats like that.
-- The shoulder position with the bar and weights is one of the best ways to manage heavy weights but you don't need that bar to do squats at light weights like the ones I am proscribing to you.
-- recommendation also includes, machine leg workouts lots of reps try to gradually add more and more weight slowly 5lbs increase or 10 lb try both. tho go for more reps instead if you can't increase the weight.
-- another recommendation if there are any good bleachers around your area to do bleacher runs , do 20 sets then run a mile.
-- variation running would include sprints, and lots of running thru your city.
-- any overpasses nearby that you could run up and down? x 25 would help. the higher the incline your running the more your focusing on your butt.
-- also, barbell lunges would also definately help, to strengthen your legs for the squat. I used to help girls out with working out so I have a little experience.
-- leg presses
-- leg curls
-- deadlifts of just the bar adding increments of 5 lbs or 2 1/2
[ Remember to make sure you know proper form to do those workouts, don't be afraid to ask for help or how to do a workout properly. Other ways are to go to your local bookstore and read up on the fitness mags to see how to do workouts properly if your shy with talking to strangers, tho anyone that is really at the gym to workout would be more then willing to help you out ]
**** You just have to go lighter that's all

If you desire to work out your legs and butt there are many many options not just squatting with the bar behind your head. Many other types of squats like the one listed with barbells.