Well, have you ever tried talking with them? That should be step 1.
If they make fun of you after a nice long talk, than, go to step 2, try to get them to like you. Find out somehow what things you both like. I'm not saying find out what everyone likes and like it, but, see who likes the same stuff as you do.
After that, if you become friends. Two things can happen.
-They'll stand up for you.
-They'll try to pretend they are not and continue ignoring you.
If it's the second one, than go to step 3, try to be kind to them and ignore them. If someones bullies you, don't make a mean come back or tell them to screw off, ask them what they see in bullying you and what good it will do.
They continue, than, go to step 4, and only do this if all the other steps don't work. This step is telling them off, as everyone says. Don't curse them or anything, that'll just piss them off and start up a pointless fight. But, confront them when they are alone and see what their problem is.
If it STILL continues, you may have to use step 5, which is talking to the principal and your parents, and having meetings with the kids parents and stuff... Although I wouldn't reccomend step 5 unless all previous steps don't work, since it may get you hated, but, if all steps don't work, you might as well do it, it's not like it'll change anything. They'll bully you either way if those steps don't work.
If it still continues, than, I guess you should seek professional help. That's all I can say.
Hope this was useful.