This exercise is mainly for your chest but your triceps and shoulders are playing a part also. There are many variations of this exercise that make it, instead of mainly for your chest muscles, to be a triceps or a shoulder exercise. I will describe the default, most famous one which improves your chest muscles.
Start by placing your hands below your shoulders in a 90 degrees angle. Let them be slightly apart from the length of your shoulders.
Your legs should be matching the length of your shoulders but if that is not comfortable for you, position them in a length that makes you comfortable. Always position them to feel comfortable.
Make your body one giant straight line. Your head should be looking slightly ahead of you.
Try not to let your elbows fly away out when you lower yourself, let your chest touch the floor and then explode back again into the primary position.
For start you can begin with 15 repetitions in a set and work your way up.