Puesto pa' ti is a popular slang phrase you'll hear a lot in modern reggaetón. It's a shortened way of saying "estoy puesto para ti".
While puesto can mean "put" or "placed", in this context it means you are completely dedicated, focused on, or "down for" someone. It's a strong declaration of loyalty and availability.
In the song, Bad Bunny uses this phrase as his main message to his partner, telling her that despite what others say, he is 100% there for her.
In "Pa Ti," Bad Bunny and Bryant Myers are trying to convince a lover to ignore the rumors about their reputation. The whole song is basically one big argument: "Te dijeron que soy un infiel / Que no debes de confiar en mí" (They told you I'm a cheater / That you shouldn't trust me), but you should believe me instead. The main promise is total devotion, with the very blunt claim that he ignores other women for her: "Le picheo a to' estas putas por ti" (I ignore all these whores for you).
Bad Bunny's verse mixes intense romance with some pretty explicit lines. He admits he's not perfect and doesn't know much about love, but he sees her as "la luz que yo veía al final del oscuro" (the light I saw at the end of the darkness). He promises to change his ways, heal her past wounds, and even offers to buy her anything she wants. It's a classic bad-boy-wants-to-be-good narrative, where he insists that while she might not be his first, she is "la verdadera" (the real one).