Start lifting heavy weights and doing squats, lunges, Bulgarian split squats, dead lifts, etc. These are good for shaping up your bum and thighs :) Mine was looking pretty flat after weight loss in 2010 but looking better now that I have been strength training for a few months.
Sure....if you want it to be composed of fat tissue.
As noted above, do the weight-training to improve muscle content.
I must say, lunges and squats didnt do much for building me a butt. I started doing dead lifts and a butt appeared in a month.