The ideal idea to set your story in the right direction is to put or imagine yourself in your story. Think of all the supernatural movies or shows you have watched in the past. What we're some of their story openers. Remember, each story should have a strong and convincing start. There should be both a protagonist and antagonist, plus the conflict.
The above said right. My line is just let the vampire live with ordinary beings, don't pull out a werewolf, but if you write werewolf ,don't write too much of their enmity. Twilight is a good series of film, but it is not a good novel. So, follow your feeling, however the book is, it will be a good novel because you write it by your heart.
Just make sure you have a beginning, middle, end and a good opener, conflict, and resolution of the conflict. Make sure you use proper gramar, spelling, and a big vocabulary. As long as you keep the storyline interesting you should be fine. And if your planning on making a series, make sure at the end you have a little bit of a hook to make people want to read the next book. About needing a storyline, just think about it and it will come to you. Make sure you right down all your ideas so you don't forget them. An author's best work, they come up with on their own. GOOD LUCK!