Take the square footage of each side of the house by multiplying the height and width of each, then add up the total exterior square footage of the house. Calculate the square footage of each window by the same process, then count up the windows, add them together, and add in the areas of all the doors. Subtract the total square footage of all windows and doors from the total  square footage of the house itself, and this gives you the area you need to cover with siding. Finally — and this is very important — you should add 20% to arrive at the total amount of siding that you actually need.