Since you've figured out the characters, how they met, if it was love at first sight or not, and the main problem, thinking of a plot should be a bit easier. However, making a story plot can be difficult work, as you usually just end up writing the same plot as your favorite book. So mix things around a bit. Here's a guideline for your plot to follow:
Introduction - Let the readers meet your main character and discover about him/her. Where does he/she live? What does he/she look like?
Initiating Incident - This is the thing that gets your story rolling. Since you're writing a teen romance, it's probably when the girl meets the guy she's about to fall in love with.
Rising Action - Where things in your story become complicated. Maybe he cheated on her? Or their parents don't want them together?
Climax - The turning point of your story. You need to let your readers know if the problems were resolved or not.
Falling Action - The main conflict has been resolved (or not) and little things are happening to wrap up the story. For example, the girl is learning to love the boy again, or her friends are supporting her, etc.
Denouement - Wrapping up the story. Letting any last details fall into place.
Conclusion - You end the story.
Note: Your denouement and conclusion don't always have to be separate. Feel free to put them together if you like.
