To create a DLL File, click on New project, then select Class Library.
Enter your code into the class file that was automatically created for you and then click Build Solution from the Debug menu.
Now, look in your directory: ../debug/release/YOURDLL.dll
There it is! :)
P.S. DLL files cannot be run just like normal applciation (exe) files. You'll need to create a separate project (probably a win forms app) and then add your dll file to that project as a "Reference", you can do this by going to the Solution explorer, right clicking your project Name and selecting Add Reference then browsing to whereever you saved your dll file.
For more detail please click HERE