A place to start.
In cell A1 type "2" (no quotation marks)
In cell A2 type "3"
In cell B5 type "=A1*A2"
Press enter. You'll get a 6 in cell B5.
From then on cell B5 will give you the product of whatever is in cells A1 and A2.
In cell D1 type "3"
In cell D2 type "4"
In cell D3 type "5"
In cell D4 type "6"
In cell G7 type "sum(D1: D4)"
Press enter. You'll get an 18 in cell G7. The total of cells D1 through D4.
Now, in cell E1 type "=G7/B5"
Press enter and you'll get a 3.
Now try changing the number in cell D2 and the value in cell E1 will change accordingly.
"SUM" is a spreadsheet function. There should be a list of functions somewhere on the page, maybe under "help". There are dozens of them. +,-,<,>,SUM,SQRT,if, and, not,... A spreadsheet is a very useful tool. One of the best ways to learn how to use it is to play with it and try different things. After a while try to use it to solve a specific task you have.
(note, two = signs in the formula bar at the top is normal)