How do i get my straight hair back again!!!!!!!!!!!!!!!!!?

i have always had straight hair but i need to figure out what i am doing wrong i need to know how to get my straight hair back again. Am i using the wrong brush. Why is my hair wavy. I take showers at night and i heard that if you tie your hair it will get wavy so i dont tie but its still wavy. When i am in the shower i have MY hair, its sheek, silky,smooth,and straight so what do i use and what do i do to make my hair straight like it used to be. please someone help tell me how to fix my hair. p.s. i never did anything to my hair to make it this way (perms ect.) only straighten it but i want it natural. i do have staright hair but i know i'm doing something wrong because nobody told me how to take care of my hair. plz if theres a proudct i use after my show or something plz tell me i'm desperate.