The answer depends on too many things that you haven't mentioned about.
Assuming that it's a regular Win32 DLL, you could use
LoadLibrary()[
^] and
GetProcAddress()[
^] or you could just use the .lib file that came with the DLL (if there was one).
If it were a COM dll, you'd register it and use it like using any other COM component.
Please search for and read up articles on both topics. Explaining them in great detail is out of context for a forum post like this.