ANDY'S ANSWER: Tenderness in your breasts is a sign that they are starting to grow and can also be a monthly symptom of your period coming on. How much your breasts actually will grow has to do with your own genetics. You can get a gauge on how they MIGHT end up by looking at your mother. But really, there's no way of knowing for sure except for wait and see what happens.
Puberty is a process that has to run its course on its own schedule for your body. Whether you start eariler or later than other girls you know or somewhere in the middle, the final result is the same. When you are finished, you will be an adult and it won't matter to anyone when it all happened.