DLL error - basic concept

I have learned how to make DLL file from the link: http://www.c-sharpcorner.com/UploadFile/mahesh/dll12222005064058AM/dll.aspx but when i build my file the error comes: A project with an output type of class library cannot be started directly. In order to debug this project, add an executable project to this solution which references the library project. Set the executable project as the startup project. What should i do to remove this error, i m just a novice, plz tell me step by step. Thank u in advance.