This week's book giveaway is in the OCPJP forum. We're giving away four copies of OCA/OCP Java SE 7 Programmer I & II Study Guide and have Kathy Sierra & Bert Bates on-line! See this thread for details.
Just about every possible operation is shown with examples in the POI documentation, including opening an XLS file, accessing a sheet, creating a new workbook and sheet, and saving it. Does that cover what you need to do?