Welcome to the Apple Support Communities
If you inserted your Apple ID when you set up your Mac or OS X Mountain Lion, just open Messages app and you will be able to use iMessage.
If not, you can check if iMessage is set up. Open Messages app, go to Messages menu > Preferences > Accounts, and look at the sidebar. You should see your Apple ID mail there and "iMessage" below your mail. If you see it, you have iMessage, and you can modify the settings selecting it