How to straighten natural hair?

I have natural 4c-4d hair, I have tried to straighten it with a flat iron and I've even searched for YouTube videos and tried what the naturals girls on YouTube do, but it doesn't work! I can never get my hair straight it just turns poofy and sticks up all around the place. So please help me and tell me the best way to straighten my natural hair and the best products to use on it.