The answer varies with the shape:  The area of a circle is arrived at by squaring the radius (multiplying it by itself) and then multiplying that by 3.1416; The area of a rectangle or square (which is really a rectangle) is arrived at by multiplying the length times the width; the area  of a triangle depends on the kind of triangle you have.  Try Google for triangle formulas.  The formulas for the areas of other shapes may be found in a basic geometry book.  Good luck!