If you had a pier which was for instance 8 feet high, 2 feet wide on and 2 feet in depth, you multiply them all together. So, 8 x 2 x 2 = 32³ft (32 cubic feet, not squared)
Model each floor of the house with one or more rectangles, compute the area of each of these rectangles, and sum them to the total square footage. If the shape of you house if really complicated you...
simply add another same size triangle to the other one only upside down. Then you'll have either a square or rectangle. multiply length by width and divide by two.
The equation is AREA = ONE HALF times BASE times HEIGHT The BASE is the length of the bottom line. The HEIGHT is the 'straight up' distance from the (horizontal) bottom line to the tip. And ONE HALF...
Multiply the length of the ceiling (in feet) by the width of the ceiling (in feet). For example, if a ceiling is 4 ft by 6 feet, the area would be 24 sq ft.