What is a good way to train before you beat up a bully?

Well first of all it depends how old you are. It sounds to me like you are around 10 to 12 maybe 9. I've been in plenty of fights to know what to do to prepare for them. First, it doesn't freakn matter if he's bigger than you cause size doesn't matter in a fight. Second, if he's stronger than you then work out and build up your muscles, don't wear yourself out right before the fight though our else you'll be to tired to win from your muscles being so sore. Third, the power of your punch doesn't all come from your arm, it also and mainly comes from the hip...so practice punching something like a punching bag(that's what i use) or a pillow...first line up your punch and then rotate your hip as you punch, its best if you do short jabs and then by the end finish it up with hard blows. Also don't look at him in the face when you're fighting him look at his chest, the reason for this is so you can use your vision to the fullest if he tries to punch you from the left or right, your peripheral vision will catch the motion and then you can Dodge it, if he tries to kick you you'll also se it coming. Looking at his chest gives you an outlook of his whole body. Fourth, 5 minutes before the fight calm your mind. This will help you out so that when you get punched it wont hurt as much as you think it would, if you think its gonna hurt alot then it'll hurt ALOT! If you calm your mind and think that it wont hurt when you get punched then it wont hurt as much. Be brave and take it like a man! I have faith in you even if i don't know you, because this has worked for me when i was 10 years old and now I'm 16 and im a one of the best fighter in my grade...O yeah and also what might help you get more strength right when the fight starts, let all your rage that you have towrds him release through every punch!