if somebody want to hail a stranger,what he can say!

"hello","hi","good morning"???

I'm not too sure what you mean by "hail a stranger." When meeting someone it's best to introduce yourself and say "hi/hello, pleased to meet you."

"Good morning" is used as a greeting in the morning. I could see scenarios where it would be used by two strangers. For example, let's say two individuals sitting next to each other on a train during their morning commute to work...One might say "good morning" so as to greet the other person and be polite. "Good morning" is also often used by friends, family members and acquaintances as a way to pleasantly greet each other in the morning.

thank you
i just want to greet somebody when i first meet on the home way!
but i find it's very difficult to me to start!

"Hello, there."

"Hail, stranger!"

It all depends on the situation. You might use "Good morning!", "Good afternoon!" or "Good evening!" as suggested by mjd, these are good in most instances. "Hello!" and "Hi!" are good too, though less formal. "Hello, there!" as suggested by Tom is quite casual and has a warmish tone to it, though I'd never use this expression because it just doesn't seem to make much sense to me ... not that that means that it's wrong.