Bike riding will do more to tone your legs...
It is not easy to build up your butt if you don't have much muscle there that can be bulked up (& most exercises are designed to tone overly large butts down...). But you want to focus on exercises that are billed as bulking up your "glutes".
Some exercise examples that focus on glutes to try:
1- Squats (from standing posit. you act like you're sitting down on an invisible chair, then raise up and repeat 10-15 times for 3 sets)
2- Lunges (from standing posit., step forward & bend knee of front leg. Either return to original position and alternate legs or keeping lunging forward alternating legs. Don't overdo or you could damage your knees)
3- Swiss ball roll down (from standing position with a Swiss balance ball positioned behind your lower back, slowly roll down to point where your thighs are parallel to the floor then roll up again. Repeat 12-15 times for 3 sets)
4- Butt blaster (from standing position use one of those exercise tubes [they look like a rubber jump rope shape] - put your wt. on one leg, hold the tube ends in both hands, and position the sole of your non-wt bearing foot over the draped part of the tube, press the leg that's on the tube back for 12-15 times & 3 sets. Repeat with opposite leg).
If that doesn't work for you there are padded panties & supports you can get that do the moral equivalent of the push up pads that go in bras - only they make your butt, not bust, look more endowed...
http://www.siliconebody.com/Silicone-Pop-Up-Booty-Enhancer
Tyra Banks has a similar offering online too... as do others.