The expression 'I'm leaving' can be said in a couple of different ways in Italian, depending on how formal you want to be.
Me ne vado is the most common way of saying 'I'm leaving', and makes use of a pronoun for both the direct and indirect object. This is very informal.
To use the equivalent of the verb 'to depart' you'd say something like 'Sto partendo' which is 'I'm leaving' although the simplified 'parto' is something you'd be more likely to hear (the pronoun io is omitted). This expression would be more commonly used if you were leaving for a specific destination, i.e. parto per la francia domani- I'm leaving for France tomorrow.
Sto anando is the equivalent of 'I'm going' and works just as well.
The exact translation of the verb 'to leave' is lasciare, and could be used if you were 'leaving' someone in the context of terminating a relationship; Ti sto lasciando! means 'I'm leaving you!' in this case. You could also use this verb in a formal context, for example if a renowned surgeon was paged urgently during a dinner party and had to excuse himself, he might utter the phrase 'Mi dispiace, ma vi devo lasciare' meaning 'I'm sorry, but I must leave you all'.