The phrase 'I miss you' is best translated into Italian as mi manchi, which is pronounced [mee mankee].
If you were referring to 'you' in plural form, as in 'missing a group of people', you would say mi mancate.
The verb mancare (to miss) is reflexive, which means it always refers to the person who is saying it.
Reflexive verbs in Italian are always preceded by a pronoun, these pronouns can apply to any reflexive verb and are tabled below:
si- himself, herself, itself, yourself (formal)
si- themselves, yourselves (formal)
In Italian, it is the conjugation of the verb (or the way the verb ends) that lets us know who is being missed. For example, if you were to say 'do you miss me?' it would be ti manco? whereas 'do you miss him' is ti manca?
To learn more about 'reflexivity' in language, why not check out this blurtit question?