Hi,

As per our understanding, your requirement is to create an routing address for creating a case. You can achive this by configuring an On demand Email to case(If you do not need to accept attachments larger than 10 MB from customers). Else go with Email to case agent.

While creating an On demand Email to case we specify an Email Id. For this salesforce itself generates routing address(service address) which has to be verified through a mail sent to the specified Email address. Once this is done, whenever we send mail to generated routing address a case will be generated in the corresponding org. 

Steps to follow setting Up On-Demand Email-to-Case
The following high-level steps outline how to set up On-Demand Email-to-Case:
1. Determine the email routing addresses that your customers can use to submit cases to your support team. Emails are automatically converted to cases based on the settings specified for each routing address.
2. Set up assignment rules, escalation rules, and auto-response rules.
3. In Salesforce.com, enable Email-to-Case and configure your email routing addresses.
You need to add a new email routing address, verify the new email routing address, and enable Email-to-Case:
Click Setup ➤ Customize ➤ Cases ➤ Email-to-Case. (More information available on top portion of this page)
4. From the Routing Addresses related list, click the ‘New’ button to create a new Routing Address. A routing address is activated after Email-to-Case is enabled.
From the Email-to-Case Settings page, you can also:
Click Edit to edit an existing routing address.
Click Del to delete an existing routing address.
Click Verify to confirm and verify the email address used in an existing routing address. A verification email is immediately sent to the new routing address, which includes a link, that when clicked, completes the verification process.
5. Enter the routing address settings and click on Save.
6. Check the inbox for that Routing Address to verify the email address
7. Add email related list to Case page layouts. Click Setup ➤ Customize ➤ Cases ➤ Page Layout and select a page layout and click on Edit. On top, click on ‘Relates Lists’ on the left panel and drag and drop the ‘Emails’ field to the page layout and click on Save
8. Configure your email system to forward case submissions to the email services address provided to you by Salesforce.com. This step varies depending on what email client do you use. You can forward them directly from your Exchange server or setup a forwarding rule to reroute these emails.
For more information, see your email system's documentation.
9. Test your email routing addresses:
a. Manually send emails to the routing addresses.
b. Verify that the emails convert to cases based on their routing address settings.

Hope this helps you !!