C# work with dll file

Question

For a C# 2008 application, I need to add a specfic dll file to an existing application and have all the project files that need to work with it reference this dll file.

Since I have never added a dll file to a solution and I have very limited experience working with C#, can you tell and/or show me how to add a dll file to a C# 2008 solution file and have all the applicable project files access this required dll file?