A girl keeps saying hi to me when i pass her or when she pass me and if dont reply she says hi even louder?

This can mean one of a few things:

One: shes trying to start a friendship with you, for one of many reasons.

or

Two: she likes you.

or

Three: her friend likes you.

or

Four: shes just really out going and trying to make more friends.

It could be one of those, or another one, but it depends on a lot of things. Like her personality, your personality, and things like that can help you tell which one.