If the DLL is accessible through standard COM then you just need to add a reference to it from your .net application.
If you are using Visual Studio then you just a reference to the COM DLL and VS will create the runtime callable COM wrapper around the COM DLL. Then you program against the DLL's api just as if it were a .NET reference. Technically it IS a .NET reference, it's just a reference to a .NET assembly that handles the plumbing to talk to the COM DLL.
If you aren't using Visual Studio, you'll need to use the tlbimp program that comes with the framework (I think, it might be part of the framework SDK). This will create the runtime callable wrapper you need to talk to COM.