Well there are actually several things that you should know. But the most important one, whether you've been writing all your life, recently started writing, or want to start writing is this:
Writing is not easy.
It drives me insane when people say it is. It's not. Take it from someone who has been doing it all her life. It's frustrating. It's time-consuming. It's irritating. Sometimes when you've been working on a project for a long time and you think it's going well, you run into a ten foot high, three foot thick cement wall. And to make it even better, there's razor wire on the top.
Sometimes you try to climb the wall, only to find that it's been lubricated with fish oil and it's impossible. And so you realize that all you can do is wait for it to fall down. I hate that wall. It's commonly referred to as Writer's Block, Lack Of Inspiration, or Boredom. And trust me, if you haven't met it yet, you will very soon.
The wall is why it took me so long to finish Uritus and the Sword of Fire. It was oh-so-very determined to discourage me enough to give up on the book entirely. But I didn't. And that's why I'm warning you about it.
People drive me crazy when they say that writing is a cinch. Mostly because I know it isn't. It may be easier at times, but it is never easy. However, if you are willing to fight the wall and press on because writing is your passion and gift than go for it. And don't let anyone stand in your way.