About or of. "I dreamed/dreamt of you." "I dreamed/dreamt about you." The latter makes for a more coherent sentence, but nobody would correct you if you said 'of'. "I dreamed/dreamt with you" suggests you and this other person were asleep and dreaming at the same time. "I dreamed/dreamt on you" suggests you fell asleep on the person and had a dream. Both of them are non-standard sentences, though, and you will be asked to clarify what you mean.