Square footage is found by simply multiplying the width times the length of the area. In the case of a roof, you may have to figure the square footage of several different areas, then add them together. If you can get up on the roof to make the measurements (being exceedingly careful, of course!) Then it can be a fairly straightforward job. You may have some triangular areas, which is a little more complicated, depending on the shapes of the triangles. Here is some advice on finding the area of a triangle: http://www.clarku.edu/~djoyce/trig/area.html
If you cannot climb up on the roof, you might get a pretty good idea about the area by measuring the area around the base of the house and using the Pythagorean theorem http://en.wikipedia.org/wiki/Pythagorean_theorem
The PT might get you the width of the flat areas but you may have to guess at the angle of the roof. This will work on simple roofs but not complex ones.
Of course you can skip all the math and just get an estimate from a roofer, which usually comes at no cost.