If you already have your SSL Certificate and just need to install it, see IIS 8 and IIS 8.5 SSL Certificate Installation.
How to create a CSR on Windows Server 2012 - IIS 8 and Windows Server 2012 R2 - IIS 8.5
From the Start screen, click or search for Internet Information Services (IIS) Manager and open it.
Click on the server name.
From the center menu, double-click the "Server Certificates" button in the "IIS" section (it is in the middle of the menu).
Next, from the "Actions" menu (on the right), click on "Create Certificate Request..." This will open the Request Certificate wizard.
In the "Distinguished Name Properties" window, enter the information as follows:
Common Name - The name through which the certificate will be accessed (usually the fully-qualified domain name, e.g., www.domain.com or mail.domain.com).
Organization - The legally registered name of your organization/company.
Organizational unit - The name of your department within the organization (frequently this entry will be listed as "IT," "Web Security," or is simply left blank).
City/locality - The city in which your organization is located.
State/province - The state in which your organization is located.
Country/region - If needed, you can find your two-digit country code in our list.
In the "Cryptographic Service Provider Properties" window, enter the following information and then, click Next:
Cryptographic service provider - In the drop-down list, select Microsoft RSA SChannel..., unless you have a specific cryptographic provider.
Bit length - In the drop-down list, select 2048 (or higher).
Click the ... box to browse to a location where you want to save the CSR file. If you just enter a filename without browsing to a location, your CSR will end up in C:\Windows\System32.
Remember the filename that you choose and the location to which you save it. You will need to open this file as a text file and copy the entire body of it (including the Begin and End Certificate Request tags) into the online order process when prompted.
After you receive your SSL Certificate from COMODO, you can install it.