It definitely takes practice and some good studying around the internet to master. YouTube has many amazing videos on writing powerful prompts, and NightCafe's modifiers menu is invaluable for learning the possibilities of what you can do. I'd also pay attention to the prompts of various talented AI artists; find what you like, don't like, etc.

There are different ways you can do it, either for free or through a website, depending on how much your computer can handle, or more so, how much time and work you want to put into it.