As it is a bit late for more tips, I just wanted to ask you good luck and assure you that you will have wicked amounts of fun.
I went there when I was 19 too, so I can kinda imagine what you feel now!
Oh just a quick thing, (dunno if someone already said it, but I don't have time to read 8 pages):
If you got the time, try riding the rollercoaster at Tokyo Dome! Isn't too expensive and is hella fun, with a nice view of tokyo