Click the Messages icon in the Dock:
In the left sidebar, if you see the name of the person you want to send a message to, click that name and skip the next two steps:
If you don’t see their name, click the new conversation icon at the top:
Then start typing their name, and when you see the person you want to start a conversation with in the list that pops up, click it:
If your text cursor isn’t in the box at the bottom already, click there, then type your message. When you’ve typed your message, to send it click the Enter key (the one with the icon like ⏎ in the middle of the right of the keyboard). Because you can’t use the enter key for a new line you can only write one paragraph at a time, but multiple paragraphs in a series of messages is perfectly fine in terms of texting etiquette.
