Squat is one of the best exercises to tone up legs and buttocks.
How? Place your feet on the ground, about shoulder-width apart, with your toes slightly pointing outward. Squeeze your buttocks one against the other and contract your abdominals so that your lower back area and whole back are straight.
To go in the squat position, bend your knees and slowly lower your hips as if you were sitting on a chair. While doing this movement, pay attention to keeping your heels on the floor and knees on the same line as your toes; your trunk will slightly lean forward, but must remain straight.
Keep the lowest position you can comfortably reach for 1 to 5 seconds, then go back to the starting position and squeeze your glutes.
Do at least 3 series of 15 squats, pausing for 30 seconds after each series.
Attention: do not lean your chest forward (nor backward) utterly, and try to bend your legs as much as possible, without stressing your knees.
Variation: to make a very good execution, you may put a barbell on your shoulders, which will help you keeping your shoulders open and solid, avoiding your trunk to lean utterly forward.