How do to say pay attention in Spanish?

"Pay attention" is said various ways in Spanish, depending on who you are talking to.

To a friend: "Pon atención."
Respectfully, to a stranger: "Ponga atención."
To a classroom full of students: "Pongan atención." (in Spanish America); "Poned atención" (In most parts in Spain)

The "o" in Spanish is always pronounced like the "o" in the Italian word "sono."