I would write "you all" as "y'all." "Y'all come back here, ya hear?" is an invitation commonly associated with speakers from the southern states of the United States. "Ya" is an informal pronunciation of "you." So if you write "ya'll," I'll think that you mean "you will."

Check these site out: http://www.urbandictionary.com/define.php?term=ya'll & http://public.wsu.edu/~brians/errors/yall.html

There is no difference between y'all and ya'll. Since this word is slang, there is no real way of spelling it, but if you are forced to write it, I suggest you spell it "y'all." However, I wouldn't say that you can use y'all for singular you, only plural you. After all, y'all is a contraction meaning "you all." This slang word is used mostly in the southern part of the United States. In northern areas it is more common to hear "you guys" or "you all" instead of y'all.