I am trying to start working with Excel documents through the OpenXML SDK Spreadsheet API. But I haven\'t found any good guides or even examples on how to create a xlsx file fro
Your best bet is to download the OpenXml SDK 2.0 and install. Once installed, check the Tools directory for a tool called DocumentReflector.exe. Using this tool you can "crack open" an existing xslx document (or docx or pptx). Once opened, the application will show you a treeview of the parts, a panel with the xml for a given part and the code required to generate the document from scratch.
This should give you a great start and get you to really understand the SpreadsheetML syntax (I used this tool to learn WordProcessingML and it REALLY helped!).
Also see how to create and download excel document using asp.net