Once you have learned the basic Excel, this should not be at all a problem. Entering a formula is very similar to copy a formula, and it always starts with an equal sign (=). Please see below for details.
Step 1: Select the cell you need to enter the formula;
Step 2: Enter the equal sign (=);
Step 3: Type value, reference or function (e.g., A1+A2-A3) in the cell.