Be careful with squats - a lot of people get hurt doing them without having had been trained properly - and with free-standing squats - where the weight is getting heavy - you should be in a gym or have a spotter. If you want to build your butt: try lunges while holding dumbbells, step- ups, extended stiff inverted leg-lifts, and don't forget to hit your hamstrings as well - or you may end up looking disproportionate. I'd also include medium weight dead lifts - both stiff and bent legged - this will strengthen your lower back as well as your upper butt.
You should do at least three sets of squats to make your butt bigger. The number of intervals in each squat can gradually increase as well as the intervals. Note that you should do very few reps with a heavy weight though this would depend on your personal fitness level.