If a girl smile and says hello and laughs a little at you when passing by and you never spoken before, Is that bad or good?

Depends on the laugh. You need to learn the difference between a playful giggle and a derisive chuckle. Women have a thousand shades of laughter and very few of them are what you might consider to be complimentary, especially when directed at males. However, if she smiled and said hello, that's your in right there. What you should also consider is that she might be a genuinely good girl and her interaction with you was completely innocent. A "have a good day", rather than a "I find you sexually attractive." Either way, buddy, you have nothing to lose by talking to her.

It's a good thing. She could either just be being friendly or she might find you attractive.

definitely good