Don't over look the help available within Excel itself. While not perfect, it does answer many common questions.
Excel's Help menu can be searched by keyword.
However, if you don't know exactly what you're looking for, try the next option on the menu called 'Excel Help'. This links to help for commonly-used tasks and features.
If you are new to Excel (or want a refresher), Microsoft offers some excellent starting points through its online Excel help centre. This microsite includes: