A formula in Google Sheets starts with an equals sign. After it comes a calculation, and you point at cells instead of typing their numbers: =B2*C2 multiplies whatever is in B2 by whatever is in C2. Change B2 and the answer updates.
You can type a cell address, or click the cell while you write the formula. Sheets adds the reference for you and outlines the cell in a matching color. Press Enter to finish.
When you copy a formula to the next row, its references move with it: =B2*C2 in row 2 becomes =B3*C3 in row 3. These are relative references. To keep one cell fixed, such as a tax rate, put dollar signs in front of the column and row: $H$1. With the cursor on a reference, press F4 (Fn+F4 on many Macs) to switch between $H$1, H$1, $H1 and H1.
Sheets works out * and / before + and -, so use brackets when the order matters: =(B2-C2)/B2.