"Sin Ti" (Without You) is a breakup song where the guy has clearly lost the argument with himself. He opens by guessing what it is he misses: "Serán sus ojos, serán sus labios" (Maybe it's her eyes, maybe it's her lips), before admitting "Pero es su cuerpo el que yo extraño tanto" (But it's her body I miss so much). Honest, at least.
He tries to avoid her and fails: "Te evito, pero te veo en todos lados" (I avoid you, but I see you everywhere). He even thinks he deserves it: "Contigo el karma ya lo estoy pagando" (With you I'm already paying my karma). Watch out for "me da pa' abajo", a casual Mexican way of saying something brings you down, with pa' short for para.
The chorus is pure hope. He wants to kiss her and, "en una de esas" (with a bit of luck, maybe), get back together. "Que tantito es tanto" is a playful way of asking what difference a little more could make, and "nomás" is Mexican Spanish for just: "Nomás dime cuándo y con un besito nos arreglamos" (Just tell me when, and with a little kiss we'll patch things up). He thinks one kiss fixes everything. Sweet, and a bit optimistic.