When you install Windows Server 2012, it will come with a 180 day trial period. After this period expires, you will need to activate the server in order to continue using it. To do this, you will need to purchase a valid product key from Microsoft or another authorized retailer. Once you have your product key, you can activate the server by following these steps:
1. Open the Start menu and type Activate into the search bar.
2. Select Activate Windows from the results.
3. Enter your product key when prompted and click Next.
4. Follow the remaining prompts to complete activation.
Once your server is activated, you can continue using all of its features without any restrictions. Be sure to keep your product key safe and secure in case you ever need to reactivate the server in the future.