Here’s how to do a perfect push-up:
From a face-down position, place your hands slightly wider than shoulder-width apart. Place your toes or 
knees on the floor, and try to create a perfect diagonal with your body, from the shoulders to the knees or feet. Keep the glutes [rear-end muscles] and abdominals engaged. Then lower and lift your body by bending and straightening your elbows, keeping your torso stable throughout.
Push ups have been named one of the most effective exercises due to the fact that it works so many muscle groups. With all exercises, good form is essential. While performing a push up, you are working your core (abs and low back), upper back, chest, glutes, and arms.
Start out slow, and modified if necessary. Perform 1-3 sets of 8-12 reps two or three times per week. Remember, take a break when you feel your form is starting to slip. Start again once your muscles have recovered.
Adapted from WebMD.com
