Why does my hair poof out when i straighten it?

when i went to get my hair styled earlier this week i did everything that she did but i cant it even close i tried using my flat iron at 345 degrees and then at 400 the next time but my just sticks out and poofs and its frustrating as hell am i using it at to high of a tempurature or to low of one....in addition i have really curly hair....plus im a guy so i dont really know what im doing so help anyone?