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.
This song is one big, confident message to an ex-girlfriend. The singer has heard the news: "Ya me enteré / Que hay alguien nuevo acariciando tu piel" (I already found out / That there's someone new caressing your skin). He knows she's telling everyone that he's the bad guy and that she's happier now, but he absolutely does not believe it. He calls her new partner "algún idiota" (some idiot) and is completely convinced she's just trying to fool herself.
He's so sure of their connection that he tells her exactly what to do. He wants her to tell the new guy, "que yo soy mejor" (that I am better) and to make it clear that she'll never truly want him. The singer's argument is simple: she's only with this new person for convenience and deep down, she misses him terribly. He ends with the ultimate prediction: "Regresarás, estoy seguro de que regresarás" (You'll come back, I'm sure you'll come back).