It's the blue chat bubble icon with a white lightning bolt inside. You'll find it on your home screen or in the app drawer (Android). If you're not signed in, follow the on-screen instructions to sign in now. It's the icon that looks like a bullet list at the bottom of the screen. It's to the right of the large blue circle. It's near the top of the screen. This displays a list of everyone who's active on Messenger. If a friend is online, you'll see a green circle on top of their profile image.

TL;DR in Vietnamese:
Mở ứng dụng Facebook Messenger. Nhấp vào biểu tượng danh bạ. Nhấn nút Active (Hoạt động) gần phía trên màn hình.