"Let's hang out": The teenage way of saying "Why we don't troddle off somewhere and hang out."
"I'm thirsty": Street slang for "Goodness me, I am indeed feeling a mite parched, and for this reason feel rather eager to achieve a beverage of some variety, in order that I might utilize said beverage upon my rather dry and brittle throat-like organ(s)."
"I have to go to the bathroom": Hip equivalent to "Mademoiselle, I wonder if I might be so bold as to inquire if I might partake of the salon which you have kindly set aside for tasks including, but not limited to, the washing of one's bodice, and the elimination of bodily fluids, through such divers methods as urination, defecation &c."
"Hi": The teen version of the phrase "Good day and however do you do, my dear Lord/Lady Buckingthorpehamshire, how are the horses? Oh la ti da, let us go gallopeth through the fields of Northamptonchestershirethorpe."
"The phone's for you": Teenage for: "Your Excellence, a communication has arrived by way of the telephonic device which hangs so stealthily from the wall. It would appear that a party wishes to engage in a conversation of some variety with your most glorious personnage, if thou wilt have it. What say ye, in relation to the matter being discussed? Yea or nay? I beg thee, most worshipful Lord, relate your wishes with regards to the aforementioned matter of the proposed telephone conversation, to I, your most humble servant, in order that I might better relate your divine wishes to the individual who currently awaits your response, like a beetle on a hook."