What are the important steps to writing a screenplay/script?

I'm a seventeen year old trying to make my way into the screenwriting world. I'm developing some ideas that are awesome movie ideas to me. However, i'm just strarting to really learn the process of writing a script and im certain there's a method to writing a screenplay. What is important, what should i do first, what order should i go in, how do i plan this whole thing out?! Anyone in the screenwriting world or a professional screenwriter PLEASE HELP!!!! I need advise.