If you're still a teenager, odds are good that you don't have enough life experience yet to write compelling characters, because you need to understand people before you can write them in a realistic way. This is something that will improve with time. That said, well-rounded, interesting characters tend to have the following characteristics.
- Goal. They should want something very badly and actively try to achieve it.
- Motivation. Why do they want to achieve this goal so badly? The more personal and specific you make it, the better.
- Conflict. What's stopping them from achieving their goal? Ideally this is a mixture of internal and external conflict. Internal conflict in particular is really important for making characters feel three-dimensional.
- Change over the course of the story. How do they grow as people in pursuit of their goal? How do they overcome whatever flaws are the source of their internal conflict?
In general, if your characters are boring, you probably need more of one (or several) of the above. Try raising the stakes and making their needs and conflicts more primal.