You can use Hangouts and start a chat from two places: Gmail and Hangouts https://hangouts.google.com/.
Starting a new chat from Gmail
1. Check the bottom left corner of your Gmail. If you can't see Hangouts, open it up by pressing the Hangouts icon pictured below.
A list of your recently used contacts appears.
2. If you can't see the person you want to send the message to on the list, click the magnifying glass icon.
3. Enter the name/e-mail address of the person you want to chat to. Click on their name on the search results list.
4. A new chat window opens up and you can start chatting.