how to start affair with this girl

So you don't want to have a relationship with her (something you have already started down the path of by talking often, playing table tennis and having juice together) but rather simply want to bed her?

Just ask her. Maybe next time you are playing table tennis, see if she would be willing to make a little wager. You know, like if she wins she gets to screw you and if you win you get to screw her. See how that goes for you.

Of course, you could just keep developing a relationship with her, ask her out on a date and let things progress naturally.