Excel can be used in so many ways, so beyond a basic understanding of navigation and how to find answers in "Help" and through the internet, I'd sugest you know what you want from it.
I'm the best in my company if you need a Macro to create something like a beta program, Someone Else is is best if you need pivot tables, charts and pretty visiuals for a presentation, Another is great if you need to import and export data to other MS applications.
Point is, this is 95% of my job for ten years now and I don't know how to do half of what it's capable of. So the best way to learn is getting help with what you need to get done at your job and network all you can so you have a base of support if you get stuck. My wife is my best resource along with the Excel help files which are very good and Google.