look up under Roddier test. Join the group, read manuals and discussions, grab yourself a webcam and you are ready to go.
http://www.astrosurf.com/tests/roddier/roddier.htm
(use web translator if your French is not that good)
http://tech.groups.yahoo.com/group/roddier/
(join in and start reading - there is also a hacked version of software with English interface instead of French)
If you want to get serious :
François Roddier, Claude Roddier "Wavefront reconstruction using iterative Fourier transforms" Applied Optics (April 1991)
Claude Roddier, François Roddier "Wave-front reconstruction from defocused images and the testing of ground-based optical telescopes". Journal of the Optical Society of America (November 1993)