Well, do you have hobbies? Interests? Passions? Do you play a sport or do some other form of exercise (hiking, biking, etc.)? Do you read? Do you know what's going on the world right now? Do you get out of the house? What kinds of things do you do with your friends? Can you hold your own in an argument?
I'm not trying to be demeaning here, but you are only 15. You still have growing up to do, and with that growing up will come self-realization and new (or deepened) interests. I was pretty boring and predictable as a 15-year-old; kept to myself mostly, stayed home most afternoons and weekends, watched movies with my friends. But toward the end of high school and then throughout college I had opportunities to try new things (like study abroad), meet new people, and take interesting classes. I'm no longer boring or predictable.
It's a process, really. Contrary to whatever tv tells you, most 15-year-olds' lives are not full of endless excitement. I wouldn't worry about it too much if I were you, and I encourage you not to beat yourself up over it.
But I will say that the most interesting guys I've ever met were guys who...
-read a lot
-were smart and able to talk about a variety of topics
-went to different social events
You could start by being more involved in your school. Maybe there are clubs, teams, or other happenings you'd enjoy. It could range from building things in woodshop to designing and painting sets for the drama club. Or you could get involved in a youth group/organization--those typically arrange lots of fun outings, and it's a chance to meet more people.