Formal:
* Écoutez-moi. (This is the most formal and direct way to say "listen to me.")
* Veuillez m'écouter. (This is even more formal and polite, literally meaning "Please listen to me.")
Informal:
* Écoute-moi. (This is the most common way to say "listen to me" in informal contexts.)
* Entends-tu? (This literally means "Do you hear?," but it can also be used to mean "Listen to me.")
* Tu m'écoutes? (This literally means "Are you listening to me?")
* Fais attention à ce que je dis. (This means "Pay attention to what I'm saying.")
Other options:
* J'ai quelque chose à te dire. (This means "I have something to tell you," and implies that you want the person to listen.)
* Tu dois m'écouter. (This means "You have to listen to me," and is a stronger way of saying "listen to me.")
The best way to choose the right phrase is to consider the situation and your relationship with the person you are speaking to.