Check out this perfect beginner exercise for isolating your butt, hips, and Legs (Thighs) and all your need is a wall and a ball.
Starting Position: Place a stability ball against a wall and gently lean against it, positioning the top of the ball into the small of your back, but making contact with your tailbone, low- and mid-back. Your feet should be positioned 6 - 12" out in front of your body, feet hip-width apart with toes facing forward or turned out slightly.
Pull your shoulders blades down and back. Do not allow your low back to pull away from the ball. Gently lean into the ball, as you shift your weight into your heels. Place your hands on the front of your thighs.
Downward Phase: Inhale. Keep the tailbone, low and mid-back against the ball as you bend your knees, and begin to lower the body. Push back with your hips, allowing them to drop under the ball. The ball will glide down the wall with you as you lower your body toward the floor. Do not move the feet. Keep your knees lined up with your 2nd toe. Continue to lower yourself until challenged or until your thighs align parallel to the floor. Hold this position briefly.
Upward Phase: Exhale and slowly push your body up away from the floor. Extend your hips to bring them back underneath your body. Continue pushing upward, returning to your starting position.
To help develop the foundations of good squatting form, emphasize dropping your hips down and slightly under the ball, and avoiding driving your knees down and forward.