Ya is how people write the sound of you when it is said fast and unstressed in relaxed speech. Nearly every English speaker produces this sound many times a day without thinking — see ya, catch ya later, thank ya, I'll tell ya, what are ya doing? — but writing it down is a choice, and the choice is casual. Where it stands for you and ("whatcha doing"), it also swallows other words, but the ya part is still you. It covers both you-as-subject and you-as-object, though the object use is far commoner (see ya, love ya, miss ya). Register is the whole story. Ya belongs in text messages, comic strips, song lyrics, and writing that copies the way someone talks. In an email to a teacher or a boss it looks careless or too matey. Spoken, it is invisible and normal; written, it is loud. A separate word to keep apart: ya also gets written for yeah/yes in some accents and in imitation of German ja. And a third one you will meet, mostly in Australia and northern England: ya used for your — "where's ya bag?" Note it has no apostrophe: ya, not 'ya.