Formulas in Excel are useful to perform various mathematical, statistical, and logical operations. You can type in a formula (though you have to be sure it's exactly right) or you can use Excel's preset formulas called functions.
If you type in the formula, you must start with an equal sign, so Excel knows that the data in the cell is a formula. After the =, what comes next depends on what you’re trying to do. If you were multiplying numbers, you would just type in the appropriate numbers and mathematical symbol (* for multiply).
The power of Excel, however, is to do more complicated calculations, including calculations for the cell itself, no matter what the content (meaning, if the content of the cell changes from a 5 to a 8, for example), and more complex formulas (such as averages, sums, etc., beyond basic math). Just like a basic formula, you need to start with the equal sign. After that, you would put the function name, then the range of cells inside parentheses, separated with a colon. For example: =SUM(B2:B5). Luckily, Excel has many preset functions, so you don't have to remember exactly what to type in – just remember the name of the function.
Since adding is used so frequently in Excel, there is an icon on the toolbar that will attempt to do it automatically. When you use the AutoSum command, Excel guesses what data you want to sum together (usually a block of cells next to the cell where the formula is).
To use the AutoSum, go to the cell where you want the summation result to appear, click on the AutoSum icon, and verify that Excel's guess is correct. If it is not correct, select the cells you wish to add to the formula by clicking on the first cell and dragging to the last one. Then press Enter/Return on the keyboard.
The sum of the data of the selected range of cells will appear in the cell with the formula. If any of the values in this range of cells are changed, the cell with the summation formula will automatically update to reflect the new sum. The AutoSum icon is located both in the Home and Formulas Ribbons.