Let's talk about apostrophes...
Use apostrophes as contractions (combining two words together):
It's = it + is
You're = you + are
I'm = I + am
Use apostrophes as possessives:
"That's Julie's dog." = that dog belongs to Julie
Exception = its (belonging to "it") "Don't touch its food or it will bite you."
You may NOT use apostrophes to indicated plurals:
"Merry Christmas from the Arthur's." - This sentence is incomplete... Who is "the Arthur?" Merry Christmas from the Arthur's what? The Arthur's dog? My last name is Arthur, therefore Peter and I are the Arthurs.