Every formula starts with an equals sign. After the = you can add with +, subtract with -, multiply with * and divide with /, like a calculator.
The difference is that you point at cells instead of typing numbers. =B2*C2 multiplies whatever is in B2 by whatever is in C2. Change B2 and the result updates by itself.
You can type a reference like B2, or click the cell while you're writing the formula. Excel colors each reference and draws a box in the same color around the cell, so you can see what the formula uses. You can also press an arrow key right after = or an operator to point with the keyboard. Press Enter to finish.
Excel multiplies and divides before it adds and subtracts. Use brackets to change the order: =B2*(1-C2) subtracts first.