Well, there certainly is a lot of questionable and downright stupid advice in this thread. Be advised, wall of text incoming.
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Righto, this is (in my opinion) what you should do (in order of application, until problem is solved):
Talk to the Older Brother
Approach the older brother and
politely ask him to talk to his brother and get him to stop annoying you. Don't get angry or raise your voice when talking to him, but be firm. It's likely he's getting some form of stick for his little brother acting like a jackass anyway, so perhaps it'd be in his best interests to slap him upside the head a little and get him to start behaving.
Ignore Them
From what you've described, the kid seems to be all bark, with very little action to back up his tough talk. By acting as if his petty attempts to goad you into starting a fight with him have no effect on you, you may cause him to lose interest and stop bothering you.
Talk to the Authorities
Now, I know this may sound like the ?coward's option?, but it isn't. Assuming you've done the above and you're still being harassed by this kid and/or his brother, you've pretty much done as much as you can yourself to resolve the situation without resorting to violence or otherwise doing anything the authorities can punish you for.
Go to a teacher, or another member of staff (if your school is anything like mine was, you'll have a guidance teacher or someone similar to go to whose job it is to deal with this kind of thing), and calmly explain the situation, starting from the beginning and giving as much detail as possible. Don't leave anything out or warp the truth to try and make yourself look better. The more honest you are, the more likely they are to believe your story, as well as make it easier for them to get to the truth once they call in the kid and/or his brother to question them about it.
After you've done that, there's not much you can do for a while except to try and continue to ignore the kid and/or his brother until the authorities either resolve the situation or fail to act and do so. It's entirely possible that being talked to by the authorities will not change either the kid or his brother's behaviour (I know the authorities were bloody useless at several of my previous schools), but it is certainly worth a try, and even if it doesn't solve anything, it proves that you attempted to settle the situation diplomatically before resorting to violence, which will make you look better in the school's eyes (as well as anyone's with any sense).
Fight Them
Now, let me stress this:
Violence should be your last resort, and only employed when all other peaceful means of resolving the situation have failed. Hopefully you will have solved the issue by now, but if not, here is what you need to know.
Firstly,
don't start the fight. It makes you the aggressor in the eyes of a lot of people, most importantly those of the authorities. Stay calm, and fight back if either the kid or the older brother engage you, but don't throw the first punch.
Secondly, regarding all of the people telling you to take a bat or similar blunt object with you (or even worse, a knife),
DO NOT DO THIS. I can not emphasise enough how stupid this is. For one thing, it turns a pretty simple schoolyard brawl into a potentially life-threatening fight, and also, it's just about the best way to mark you out as the bad guy in the eyes of both any spectators and the authorities. It proves that not only were you deliberately trying to grievously injure another person, but also that you we planning in advance to get into a fight. Again, do not do this.
Judging from what you've said, the little brother shouldn't be too hard to deal with, since you're older than him. You could probably quite easily beat him. Just remember that a lot of people will (and rightfully so) look down on you if you hurt him any more than is necessary. Especially the authorities. You're older than him, you're bigger than him. Shove him away if he tries to fight you, don't beat the crap out of him. It's the bigger brother that'll be the problem, assuming he even gets involved. If you do get into a fight with him, try and ensure that it happens in a fairly open area, where there are a lot of other kids. This will provide a large number of potential witnesses for the authorities to question later, and the more witnesses that see you defending yourself in a fight rather than starting one, the better.
Try and hit him in a way that will either wind, disorientate or knock him down. Don't go for the throat, because you don't want to risk killing the guy. Stomach, gut, temple and (though some people may disagree) testicles are valid targets. Hit him until he either backs down, gets knocked down or otherwise stops hitting you back. Do not hit him again after this. It makes you the aggressor rather than him, and is entirely unnecessary.
Walk away, and either go to the school nurse or whatever if you have any injuries and report what happened to the authorities, or carry on as you were (if he started the fight and lost it, he's probably too embarrassed to go to them himself). Also, it's worth noting that you may not even have to win the fight against the bigger brother to get them to leave you alone. Just stand your ground, and even if you end up losing, you'll probably gain a few points of respect in his eyes, or at least get him to try and find an easier target.
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Now, that's hardly a perfect or complete set of advice, but aside from that, I'm not sure what else I can offer, aside from don't take your high school 'reputation' too seriously. It's not nearly as important as it may seem to you at the moment, and acting like a responsible person will earn you a lot of points in the eyes of the authorities as well as those of any of your peers with half a brain.
Hope things work out well, in any case.
Pointer said:
Do what Ender did. Beat the ever living shit out of him in front of this friends. Kick him when he's down. All that jazz. Then make a threat that even if he gets his brother in on this, even if he gets all his little shits together to beat on you, one day you will find the other kids alone and wail on them like there is no tomorrow. The problem with that is you have to make good on your promise and you look like a ruthless bastard.
The way I dealt with this when I was 13 was to Suplex the guy by the head. Didn't bother me ever again.
I hope you're kidding. Please tell me you're kidding. I'm not sure if you read the same book I did, but Ender
killed Stilson. Are you really advocating that the OP commit murder because he was shoved around and threatened a little?