If the person you want to chat with is listed as a recent contact, you can simply click on their name to show previous conversations and to send them another message, otherwise you can search for peopel in your organisation, or by email address.

Click on the + button to the right of Chat, then click in the People, space or bots box to search for people you have not yet started a conversation with. 

As you type, suggestions will display. 

Click on the person you wish to start a conversation with.  If you haven't chatted with them before, a blank chat window will show, with a text box along the bottom of the screen.