SPFolder related operations in SharePoint

1) Get SPListItem(s) of a particular SPFolder

SPList splist;
SPFolder spfolder; //Get the required folder instance
SPQuery spquery = new SPQuery();
spquery.Query = string.Empty;
spquery.Folder = spfolder;
spquery.ViewAttributes = "Scope=\"Recursive\""; //For nested folder structure

SPListItemCollection itemCol = splist.GetItems(spquery)

2) Create a SPListItem based on a custom ContentType or SPFolder ContentType

SPWeb spweb; //get the SPWeb instance
SPContentType myCType = splist.ContentTypes["Custom content type"];
SPList splist = spweb.Lists["My SPList"];
SPListItem splistitem = splist.Items.Add(); 
splistitem["ContentTypeId"] = myCType.Id;
splistitem.Update();

splistitem["Title"] = "New Item";
splistitem.Update();

3) Add a SPListItem in a particular SPFolder

SPWeb spweb; //get the SPWeb instance
SPContentType myCType = splist.ContentTypes["Custom content type"];
SPList splist = spweb.Lists["My SPList"];
//get folder instance
SPFolder spfolder = splist.Folders[1]; 
SPListItem splistitem 
   = splist.Items.Add(spfolder.ServerRelativeUrl
                      , SPFileSystemObjectType.File, null);

splistitem["ContentTypeId"] = myCType.Id;
splistitem.Update();

splistitem["Title"] = "New Item";
splistitem.Update();