It's possible to change your body, a little, with work. I bet you look fine though. Yes a lot of guys seek a woman with a curvy behind but many like a smaller one. Your slim so I bet it fits your body. Lots of girls would love to just be slim.
You're not fat enough. Literally. Strong butt muscles do lift but the size is generally related to fat.
So your only workout is to workout and eat a bunch at the same time keeping the top toned and the bottom ... well neglected.
If you're slim, you're fine. Stop worrying about your butt.
Squats! Buy a barbell and some weight plates (Not too heavy) start slow and work up.
Do a squat workout and also lunges. My friend did the 30 day squat challenge and hasn't stopped since. She posted a picture of her butt after doing it a good 4 months and she has filled out a lot in that area.
Eat well too...plenty protein and consider protein shakes :)