Simple enough, go to a nightclub, there will be young girls in there.
Make some friends and chat with some girls, be confident and kind.
In Japan the subway is closed between IIRC 12-5am so if you are in a nightclub after 12:05am its quite likely that most of the girls there are looking for it. (It is a nightclub after all)
Another quirk of Japan if you have a group of say 3 girls, if they don't all get laid, non will get laid. So you will need to make sure her friends are also being touted for - work out who they are and make friends yourself with some decent blokes in there.
If you want to avoid this if you can find a girl by herself (or perhaps just one other friend), chances are high they are looking to get laid. Bigger groups less so.
Then take her to a Love hotel nearby at the end of the night or wherever.
Just be confident mate, might take a few attempts but its not as difficult as you might think. Make sure to have a good time!