If you want to tone up and strengthen your butt there is no substitute to squats.
To perform squat:
Stand with your feet shoulder width apart, directly under your hips, and place your hands on your hips.
Standing up tall, put your shoulders back, lift your chest, and pull in your abdominal muscles.
Bend your knees while keeping your upper body as straight as possible, as if you were lowering yourself onto a seat behind you. Lower yourself as far as you can without leaning your upper body more than a few inches forward.
To make squat more challenging use dumbbells for added weight.