When I straighten my hair it never looks like I want it to help please???!!!?

So I have a chi medium sized hair straightner and whenever I straighten my hair it never comes out looking really good only ok I have no idea how to make it look good all the girls at my school have there hair perfectly straightened and mine never looks like that I mean are there like any products I need to use or anything also how do you straighten the back of your hair I can never reach it please help any tips you have would be wonderful and if you could say what you do everyday that would be amazing also if you want you can tell me the brands of the products and straightners you use I really need help