It might be really hard to see it at 15, but what happens in high school matters so little in the real world. How popular you are doesn't matter. Your reputation does not matter. Really, the only things that matter are your grades and your own well-being. If/when people pick on you, brush it off. If they keep it up and it is bothering you to the point where you are so troubled that you are crying at home about it, tell your parents and school administrators and ask for help. Do not try and handle the situation by yourself when you are obviously in a weak emotional state.
Now with that out of the way, it also sounds like you have some self-esteem issues. You have come to this forum, so I am going to infer that you have a desire to feel like you can defend and stick up for yourself. I suggest that you start getting involved in some sort of martial arts training. It will not only make you more confident in your ability to defend yourself, but it will also give you something to be proud of as you improve. If your high school has a wrestling program, that would be a perfect way to start. If you would rather do something not associated with the school, check out the local martial arts instructors in your area.
Whatever you do, just remember that the crap people are putting you through now is not going to mean anything in a few years. Keep your chin up (unless someone is punching you... then you need to tuck your chin
), learn to respect yourself, and keep your eyes on what is important.
Life is constant change, and this too shall pass.