Why did she give you juice? Sounds cute, but a bit odd, giving juice to randoms.

Anyway. It's hard to gauge how/when you should approach a girl until you actually get to know her. Most girls, as >>13 said, like the moment of mystery surrounding the status of the relationship. It's sweet, fun, and as much as I hate to admit, we girls do like the thrill of the chase. When girls find out that a guy they've recently met likes them, the ball in their court, which leaves you on the back-foot. Many a times (not all), this situation has produced toy-boys. Hence I suggest subtle hints, a smile here and there to charm her and see if she responds favourably. Don't overtly suggest anything to her, just tease her with a compliment here and there, ask about her interests etc.

I think that's the thing with guys approaching girls and failing. They're too forward. If you hardly know her, and come down hard on her when you first meet her, who's to say you wouldn't do that to another girl? If you get to know her better before asking her out, then there's more chance that she'll think you're genuinely interested in her.