I think the best way to get working with the various functions is just to try them out. The online help has lots of great information including examples of pretty much every function that are easy to work with.
First become familiar with the expression editor and how it can help you build expressions.
Then, with a data set that you are familiar with, try out the different functions listed in the help to see what they do. Most are very straight forward to get working. It's ideal if you have a data set that has some string data with a few dimensions as well as numbers so you can try out many different things.
If you are an Excel user that is accustomed to doing more advanced calculations then you can try to mimick something you've done using it previously, many of the capabilities and syntax are similar in some regards so it can be a good starting point.
If you get stuck trying to do a particular thing and can't find the answer already here on community (where just about every question has been answered before regarding functions) then post a question describing where you are stuck and we'll help.
