My second suggestion would be not to worry about what people will think, either about your genre choice, or the fact you're reaching for a pretty high goal. Write what you want to write, because if you don't love it, no one will. If you want to write about witches and vampires, do it. The next great American Novel? Go for it. Romance? Why not! You're not the only one who loves these genres, so even if your friends think writing paranormal books are lame, there's an entire community out there who does not!
Lastly, find a process that works for you. There's no right way to write a book. Some writers like to plot the whole thing out, and some people like to just sit down with no idea what they're doing and go for it (I happen to be the latter). If you're not sure which works for you, try them both. You might even find the process is different for each book you write. Basically, don't think you can look up a "how to" with this. You aren't building furniture from Ikea. It's okay to be unsure how to start, or finish. Just trust your gut, and if later on you don't like something you wrote, that's what the backspace button is for.