Formula

Hello Mai, welcome to the forum, thank you for your question, in answer, type the equals sign (=) in a cell and you are on your way, if you simply wish to calculate the Sum of two values, then the formula =1+2 is sufficient, press Enter, or leave the cell and 3 (the answer) will appear. If you use Cell references, =A1+B1 this will give you an answer (say in C1) of the two values in the two cells.
Consider the Acronym BODMAS
This is the order in which Excel calculates a formula;
B = Brackets (parenthesis)
O= Ordinals (raised to the power of)
D= Division
M= Multiplication
A= Addition
S= Subtraction
To go a little deeper; Double-click on the cell where you want the total to appear.
Try the steps below for an example;
Enter an opening parenthesis character: =(
Enter a cell reference, for example: =(E2.
Press the + key.
Enter another cell reference and a closing parenthesis character: ) for example: =(E2+E3).
Enter a minus sign and a third cell reference, for example: =(E2+E3)-E4.
Press the Enter key to accept the formula.
The cell will display the sum of the first two cells minus the third cell.
If you want to learn more about Formulas and Functions, click the fx (Insert Function) button on the Formula Toolbar, you can then cycle through all the options, be aware there are enough different functions and formulas to keep a Mathematican happy. I hope that has given you a start, if so, please click the Resolved link, best regards Pete.