Navigate to the bottom menu and locate "Chat" and click on it to enter.

Locate the “+” icon in the bottom right corner to search users you will like to send a message;

Enter the recipient's username and compose your message.

 Keep in mind that there is no character limit to sending messages. 

You can also send images and files to other users in chat. 
Locate the “pin” icon at the bottom left and click on it;
Browse your device to send images and files via chat.