Ya me enteré translates to "I found out". It comes from the incredibly useful pronominal verb enterarse, which means to find out about something, often a piece of news or gossip.
This phrase is the song's title and opening line, immediately pulling you into a story of heartbreak and drama. The singer has discovered his ex is with someone new, and the entire song is his confident, emotional reaction to this news.
Mexican pop group Reik does not hold back on the drama in Ya Me Enteré. The singer found out his ex has moved on with someone new, and he is not handling it quietly. Instead of wishing her well, he flat-out insults the new guy, calling him "algún idiota" (some idiot) and demanding she say goodbye to "ese perdedor" (that loser). He knows the rumor mill is spinning against him, admitting, "soy el malo y todo el mundo te cree" (I'm the bad guy and everyone believes you). Still, he refuses to accept that she is actually happy.
His ego takes the steering wheel for the entire chorus. He claims she is only with this new partner out of sheer convenience ("por pura comodidad") and is completely bored in his arms. When he sings "eres mi otra mitad" (you are my other half), he uses a classic romantic idiom to insist their bond cannot be broken. It is petty, extremely confident, and full of jealousy, but he genuinely believes she is faking her new romance and will come running back.