COM objects can be created in several different ways and with different tools, such as Visual Basic or C++). Several applications expose functionality as COM objects - this includes most of Microsoft's applications (Office, Internet Explorer, etc.) and many third-party applications.
If you want to call an external application that exposes itself using COM, you need access to the COM documentation provided with that application. For example, if you wish to call Microsoft Excel components, you will need to understand Excel's object model, which is documented in the Microsoft Developer Network Website.