Normally, if you need to use a COM object in C#, you would add it as a Reference, and select the registered type library. That will generate an Interop Assembly, after which you can use the COM object just like any other C# class.
Alternatively, you can run the .NET utility tlbimp by hand, which has roughly the same effect but gives you slightly more control.
If you really need to create the object dynamically, without knowing anything about the type ahead of time, you can use the dynamic keyword and the Activator class to create a dynamic instance of a type. The code would look like:
var comType = Type.GetTypeFromProgID("some.someClass");
dynamic obj = Activator.CreateInstance(comType);
This will defer all type checking on obj until run-time, behaving much like VBA would.