Squat with weights 2-3 times a week. Do low repetitions (say 4 sets of 6 repetitions) with as much weight as you can lift. Make sure to do it right to not hurt your back and knees!!
Take protein shakes before and after working out.
Eat! You need to feed yourself or else you'll just have a hard butt, not a bigger butt! I work out but sometimes I neglect myself and don't eat enough and my booty decreased an inch in size
Try fish oil: rub two open capsules of fish oil on each butt cheek and wrap your butt with saran wrap. Some girls gained 4 inches doing that in 2 months! The only fish oil that I know that doesn't smell bad is the Spring Valley fish oil 1200mg that you ca find on walmart in a pack of 2.