Astral projection is basically when your body is sleeping and your mind is awake. At that point you start to change vibrations, and enter the astral relm. Which is pretty much a dimension of higher vibration. At least this is my basic understanding of it. So the first step is to enter a deep meditation. This is hard to achieve unless you are already adept at it. So to help you may find guided meditations for AP on YouTube. These will put you into a deep period of relaxation, ordinarily you would fall asleep. Although if you want to astral project you must keep your mind awake. You can do this by repeating "mind awake, body asleep" in your head. When your body falls asleep you will become paralized. Do not be afraid though, it is normal. It is VERY IMPORTANT you remember not to be afraid. Once in the paralysis state you need to focus on 1.Staying concious, 2.Image yourself leaving your body. If you stick to it then the next step is the vibrational stage. You will feel a kind of electricity moving through your body, this is the final step. Once in this stage you need to focus on your desire to leave your dense body, to be free. If you are 100% ready, your astral body will manifest.
The above will not happen on your first try, its definately a learning experience. so take time and set a goal, try and meditate X times a day. Eventually you will have your first astral experience. Oh, and dont forget to do more research into the topic. I cant possibly give you all the information you need within a paragraph, so watch YouTube tutorials! Good luck!