How to measure the roof?

Simplest way measure your roofs square footage even take a picture so they can see if what kind of roof it is and if you have dormers, or gable ends etc. Then take that number and pic down to the hardware store, or where ever your buying material and they will load you up with the right amount of paper and shingles plus they might even give you tips on how to do it because it sounds as if this is your first time so don't be shy ask for a quick demo because its important to start correct on the roof or it ends up looking like @!#%$