What's the best thing for you? If you like to have a really vivid image of what you're going to write, you're probably an outliner. In my opinon, all novels need some type of outline. The amount of detail you put into the outline, though, is up to you. It all depends.
First, let me say, I've never completed a novel. I'm way too young to dedicate my time to writing. When I finish school, I'll be ready. So, first, be ready to take the time out to write. =)
Next. A teen romance novel ... If teens and romance are subjects you know well (from personal experience) and remember well (if you're not a teen at the moment), then the story will come from your heart. The most detailing in an outline you'll need to do will be characters and events. But the emotion behind it will come from you.
Research anyway. If your setting is a real location, research it. I like to make up cities, however, but I still need to be aware of the weather if the cities are north or south or east or west.
And don't make it a job. Put your heart in everything you write. Outlines are helpful, but don't restrain yourself to them. That's why this is "creative" writing. It's all you. The outline is like an assistant.
Best of luck!! Tell us when you're published!! (Teen romance happens to be a favorite of mine, so I'll definitely be interested.)