How to create DLL file

OPen VS, and either select a solution, or create a new one.
If you pick a solution, then open the Solutions pane, and right click the solution name. Select "Add ... New Project" from the dropdown.
In either case, In the templates dialog, select "Class Library (.NET Framework)" and press "Next"
Give it a name as usual and create the project (for a new solution, you will probably need to add a "Testing" project to provide an executable that can call your DLL assembly classes, add a reference to the DLL project to it, and make it the "Startup project" - DLL files cannot be executed directly).