[Hatix]

IMPORTANT NOTE: you cannot buy a JR Pass in Japan! You have to buy a voucher for it in your own country and then you can exchange it for a JR Pass in Narita or a big JR Station. Check the english website of the JR Pass for addresses: http://www.japanrailpass.net/eng/en005.html

Also: also try the reseller shop in Ueno, it's bigger than the one in Harajuku and sometimes has more choice. I also suggest taking a hostel in Tokyo, not in Yokohama, you'll probably only be in Yokohama for the concert (and maybe visiting Minato Mirai 21). And you would have to get on the train half an hour every time to go to Tokyo, and then take other trains/subways to other parts of Tokyo. I took a look at hihostels and they don't have much choice for Tokyo, try hostelworld.com as well. Compare and see what you think is good. It's always best to check a few different sources.
Also, plan ahead what you're going to do in Tokyo, don't go there and think "we'll see what we do". Tokyo is big and so that you know where everything is and how to get there (which train/subway). Because it's big, it's not like just strolling through Tokyo, you don't do that. You stroll through parts of Tokyo which are 10-20 minutes from each other . Not to scare you of the size, the train- and subway system can get you everywhere in no time.
It's just that personally, I want to be prepared before I go somewhere I haven't been. Things like size, distance, language barrier... if you have (almost) everything planned out up front, you don't need help on the spot. (Although the japanese people are very friendly and would love to help you out)

P.S.: I think I'm really gonna have to make a big tutorial too, with some travel tips and how get things like a Japan Rail Pass. Not that I know everything (certainly not!), but that I can share the things that I do know.