Heheh clubbing in Japan is a DREAM dude.
Endless girls that LOVETH the foreigner, cheap drinks etc.
In Shibuya tokyo find a place called Club Pure. Only open Thursday, Friday and Saturday. 1000yen at the door for all you can drink all night. Hip hop and R&B music mind you, but also some SSCORCHING hot womens.
Gaspanic. Hmmm Shibuyas is a rathole. Ropongi is TOO MUCH of a meat market.
What you want is Yokohama Gaspanic on a thursday night. All drinks are 400yen. A stunning collection of girls. Ok music. Oh and their comic 'You MUST have a drink in your hand to stay in Gaspanic' rule.
I honestly say avoid Ropongi. Its meant to be the party capital of Tokyo, but Shibuya and Yokohama are MUCH better.
Remember odds are you'll be doing all nighters as the trains finish running WAY too early. But when the booze is flowing and the women are grinding is that a problem?
Only 19 but look over 20?
I'm 23 and look 18 and I have never, EVER been carded in Japan. I know a 15 year old American girl that is a HUGE clubber. Don't sweat it man.