Start with your feet approximately shoulder-width apart...

Muscles used :
Quadriceps (front of thighs)
Hamstrings (back of thighs)
Gluteals (bottom)

 
1.    Start with your feet approximately shoulder-width apart. (You do not need to hold any weight. Start out with your body weight until you are accustomed to this exercise. If you are an advanced trainer or wanting to make the exercise more difficult - then hold some weights).

 
2.    Keeping your back straight, breathe in and bend at the knees. Stick your bottom out and squat down as if you were going to sit in an imaginary chair. Make sure your knees are going straight over your toes. 

If you are a beginner or have a knee injury, only squat down as far as you feel comfortable. ie. begin with small movements and as you become more flexible and accustomed to the exercise, start to squat lower each time you do this exercise. If at any time you feel any sharp pain under your knee cap, stop doing this exercise and consult your health professional. 

Breathe out as you return to the starting position. This is one full repetition.