OMGG... you're sooo luckyy your doing. Have fun anyway.
Well, you wouldnt really wanna wear something so catchy and flashy cause you might freak 'em out. Just wear something cute but casual at the same time =D.
For example something like these skinny jeans would be great :
http://www.abercrombie.com/webapp/wcs/st… or this black one
http://www.wetseal.com/catalog/product.j…
(I heard skinny jeans are the best for concerts)
And you can wear a cute top like this :
http://www.wetseal.com/catalog/product.j… (Ok... this one is flashy but its still cute ! lol). Or just wear a tee like this :
http://www.wetseal.com/catalog/product.j… etc.
And some flats to go with it, like this :
http://www.forever21.com/product.asp?cat… or this :
http://www.wetseal.com/catalog/product.j…
Hope I helped... Ooh and yeah suree, you should totally make a cute bright colored poster (they said in an interview that they liked it when girls do that so... =P haha). Like you can put their picture on it and color the sides with bright colors etc. and you can put quotes like : '' J.B Rock My Sox '' or '' Everyone Wants A Prince Charming... I Want (( 3 )) '' ! LOL... of something like that.
Good luck and most importantly, HAVE FUNN and ENJOYY the show <33