//Create folder: 
SPDocumentLibrary docLib = GetYourDocumentLibrary("YourSiteName","Pages");
SPListItem newFolder = docLib.Items.Add("",SPFileSystemObjectType.Folder,"first folder")
newFolder.Update();
//Create subfolder: 
SPDocumentLibrary docLib = GetYourDocumentLibrary("YourSiteName","Pages");
SPListItem newFolder = docLib.Items.Add("http://yoursharepointsite/yoursite/pages/first folder",SPFileSystemObjectType.Folder,"first folder")
newFolder.Update();

Only thing needs to mention is Items.Add() function. First parameter is url, when create folder at the root, use "", when create subfolder, you have to provide the full URL including site collection, site, document library name and parent folder name. It took me 1 hour to figure out, hope this post will save your time.