Google Sheets Course: Functions
There are several hundred more or less complex functions in Google Sheets, some of which will be very useful to you.
Insert a Function
To insert a function, enter a = then the first letter(s) of the function to display suggestions, and press Enter to validate your choice:

In the case of the SUM function, select the range of cells for which you want to get the sum and validate:

To choose a function from a list, click on in the toolbar or on "Function" in the Insert menu:
You will notice that a short description is displayed when hovering over functions.
In the case of the AVERAGE function, also select the range of cells for which you want to get the average and validate:

More Functions
You will find explanations and examples of usage for the most common functions on the page dedicated to functions (accessible via the "Functions" link in the top menu).
