To find out the names of the entrypoints in the dll, type quikview X:\winnt\system32\comdlg32.dll and page down to the "Export table".

To get further info on the apis themselves and how to use them, try google. The information on the first hit at the codeproject looks pretty useful.

MSDN usually turns up a reference for the API, but don't ignore stuff at other places, nor example code in VB/VBA, often these have better explainations of how to use the apis.

Good luck.