If you want to have a sleeker thighs, you will have to do several exercises. The first exercises you should do are those for beginners.
Bent-Knee Extension targets back of the thigh (hamstrings). You should start lying face up on the floor, knees are bent 90 degrees, arms are at the sides and heels are in shoulder width on a stability ball. Lift the hips from the floor, hold 3 seconds and lower them. Repeat this 10 times.

Seated Leg Raise targets the front of the thighs. Lie on the floor while your back are pressed to the wall. Lift one leg, as much as you can and lower to the floor without touching. Do 20 repetitions for each leg.

Scissor Kick targets adductors. Lie on the floor and lift your legs up to 45 degrees. Quickly open them, crossing the left over the right thigh then switch. Do 12 repetitions.

These are intermediate exercises.
Extended Leg Curl targets your hamstrings. Lie on the floor, while your legs are on a stability ball. Arms are on both sides. Lift your hips and wait for 2 seconds. Pull the ball towards you and push it away, while your hips are lifted.

Single-Leg Wall Extension targets quads. Sit on the floor and press your back on the wall. Bend your knees 90 degrees. Lift one leg and hold it 3 seconds. Do 10 repetitions and switch legs.

Ballerina Squat with Knee Bend targets your adductors. Stand while your feet are wider than shoulder-width. Bend your knees 45 degrees and stay in that position. Switch your body weight to the left side while straightening right leg. Hold that position for 1 second and then switch sides. Do 10 repetitions.