It is really simple. If you have a domain controller in your organization, joining a Windows 2012 server to your existing domain is similar adding a Windows 2003/2008 server
Log in the server with local admin account and change the IP address and give it a static IP address.
And then open Server Manager Dashboard and go to Local Server
Click on Workgroup and then Change button
Choose Domain and type your domain name and then click on OK button.
When you type AD administrator username and password and click ok, your server will be added to domain