In order to send a message to a profile, you must first have "liked" it and the profile must have expressed a mutual interest in you by "liking" you as well. 

Communication between two profiles is based on mutual interest

Once the mutual interest is validated, you then have a match. By clicking on your matches you start to chat. 

To access the Chat and the list of your matches, from the home page of the app and in the top navigation bar, please click on the chat icon :

On the new screen that appears are the profiles you can chat with. If no profile is displayed, your profile has no matches yet.