Overall most people tend to learn better this way rather than trying to learn all Excel formulas at the same time as this can be very confusing. Most of us tend to learn in smaller packages of information and you should look for a course of instruction that will allow you to do this. As you are learning to use Excel you should of course start by learning to use the formulas that you are actually using or are likely to have need of in the immediate future.
What‘s the difference? As with the open-source website editors, it gets a little confusing. Both LibreOffice and NeoOffice run on the Mac and both are open-source. The interface of both look almost identical at first glance, and they both share largely the same features. So why the redundancy and which should you choose? Well, while LibreOffice runs on both Mac and Windows, NeoOffice is Mac only. It is a special branch or fork of OpenOffice that was developed to quash some of the bugs in the Mac version of OpenOffice. So I would say, if you‘re going to choose one, choose NeoOffice.