I need to sign out of iMessage on my Mac. How?

Do the following:

1. Open up Messages on your Mac
2. Go to the Message Menu and select 'Preferences'
3. Select the 'Accounts' Tab
4. Choose iMessage
5. Under Settings, choose 'Sign Out'

Open the messages app.

Open the preferences (Messages>Preferences)

Go to the accounts tab.

Either disable it or sign out entirely using the "sign out" button.