If you're using the Messaging app for the first time, you first need to add your contacts. You can do this in two ways:
Connect your Facebook account
Swipe in from the right edge of the screen, and then tap Settings.
(If you're using a mouse, point to the upper-right corner of the screen, move the mouse pointer down, and then click Settings.)
Tap or click Accounts, tap or click Add an account, tap or click Facebook, and then follow the on-screen instructions.
Invite people to Messenger
Swipe up from the bottom edge.
(If you're using a mouse, right-click within the app.)
Tap or click Invite, tap or click Add a new friend, and then follow the on-screen instructions.
To send a message:
Enter the first few letters of the name of the person you want to chat with and press the plus icon
Tap or click a person from the list of contacts that appears.
Type your message and press Enter to send it.