I always say you float first and swim second. Let the water work for you and not against you. Recall how a hot dog is a piece of meat with no lungs and it still floats in a pan of water. So unless you only have 6% body fat which I doubt – you can float by fully inflating your lungs and relaxing your muscles lying flat on the water surface (just like stretched out on a mattress or floor).
Keep your mouth open to make a better air lock with your nose. Watch the video to see several ways to teach yourself to float. Hint: lie down stretched out on your mattress or floor first, then do the same on the shallow water first step into a pool. But keep your forehead down looking at the bottom and keep your mouth open holding your breath for at least 15-20 seconds so you can feel yourself floating. You must feel your upper arms pressed against your ears or your head is not looking down enough. (Note: if you insist on lifting your head up forward your legs will have to go down and you will not float level like an air mattress.