GetSaveFileName Parameters

I can't figure out how to use this function. Any help would be greatly appreciated.

I've got this so far: But how do I make the dialog appear?
OPENFILENAME filename;

LPCTSTR Filter = "*.txt";
LPSTR InitialDir = "c:\\";
LPSTR DefExt = "*.txt";
filename.lpstrFilter = Filter;
filename.lpstrInitialDir = InitialDir;
filename.lpstrDefExt = DefExt;
OPENFILENAME *filepoint = &filename;
GetSaveFileName(filepoint);