You can use it while speaking but never in writing. Ya is american slang, I dont know the rules because it is slang and doesnt have rules >.>

As a general rule - 'ya' is very informal, 'you' is normal/formal. I wouldn't ever use 'ya' in writing unless I writing a short message to a friend. Don't use it in a job application or anything important! In speaking, I wouldn't use it often at all. To me it sounds like something more common in Australian or some American accents.