Arduino Canon Hack control

Hi everybody
Im new with Arduino and I dont remeber too much about programming.
Im a scuba diver and a photographer and Id like to build my personal underwave photo huosing.
Id found something interesting about the DIY housing but I need some help about programming the Arduino.
Could someone tell me which are the basic command to open a session with a Canon Eos and change Mode (M,Av,Tv,P,A), speed, shutter, iso and exposure compensation?
Regards

Luigi

What EOS do you have?

I have a Canon 20D and a Canon 550