Microsoft Excel: On-line Course Recommendation

First piece of advice: if you can help it, don't use a Mac. Excel for IOS is a much inferior product to the Windows version - less functions, capability, and most importantly lack of keyboard shortcuts (if you're going to be good in Excel, you should never have to use a mouse - keyboard shortcuts is the path to efficiency).

As for a course, it depends what you are trying to do. Financial modeling? VBA? Statistical analysis?

Training the Street is the probably the best Excel training I have ever taken (focused on financial modeling but all spreadsheet based). I did it in person but I'm sure they have online courses.