Straightening Tips NEEDED! How do you straighten your hair perfectly? How do you get it to stay?! HELP!!?

I have gym first period. and my hair is pretty staright in the morning, but after gym its a reck! how do i get my hair to stay straight all dayy? And how can i make it striaghter then it is..Cuz i have naturally wavy hair and no matter what i do, theres a little wave in it. Help me please! Thanks!