French, the same as english, has the formal and "slang." Here are some quick slang words when saying hello to friends in Paris.
You probably should not use French slang with strangers especially if you are over 30- when in doubt stick with the classic "Bonjour". Once you get to know someone a little better, then feel free to use the slang words provided!
Hi- Salut (Suh-lude) soft "d"
Yo Dudes/guys!- Oh, les gars! (o- lay gare)
Yo girls!- Oh, les filles! (o- lay feey)
On the telephone:
Hello?- Allo? (al-lo)
Hey!- Salut! (suh-lude) soft "d"