Finding information about functions can be difficult, especially if you do not know the name of the function, or if you are trying to find a function that will do what you need. Two ways to find help with functions come directly from Microsoft:
First Option – If you know the name of the function, but are unsure how it works or what is required to make it work, then you can use the Insert Function command for help. To access this function, click the Formulas tab, then the fx Insert Function command, located to the far left of the ribbon.
This will open a dialog box. Type the name of the function you are trying to find in the Search for a Function field, then click Go.
In some cases, you can type in a description of what you are looking for, but it might not give you the correct function.
Second Option – It is always a great idea to know what is available to you as the user, or have an idea what functions have been added to new versions of Excel. The easiest way to do this is to do a search in Help.
To bring up the Excel help dialog box, press the F1 key on your keyboard. Once the dialog box is open, type Functions by Category in the search field; the first option should read Excel Functions (by category). Click the link and all categories for functions will populate.
Click the category you wish to view, and the functions from that category will populate. Once you find the function you want to view, click the link and you will be able to view details about the function, including when and how to use the function, examples for setup and outcomes that function will give you, and, in some cases, videos for setup of the 
function.
