"Cię" is a short form, and "Ciebie" is a long form.
You should use short form generally. But there are few exceptions:
- use long form in the begging of sentence, e.g.: Ciebie nigdy nie ma w domu.
- use long form to emphasise the person you are talking about: Kocham ciebie, a nie jego.
- use long form with prepositions: List napisany przez ciebie.