Well the pinouts are fairly simple. Here is a list of the pinouts of the various camera's cables: www.doc-diy.net :: camera remote release pinout list
As you can see, the 2.5mm cable has 3 wires. The one closest to the cable is the ground, the middle one is the focus, and the tip is the fire control. If you connect the ground wire to the middle wire, it should cause the camera to focus (do the 1/2 press of a shutter release). On some cameras, you will need to connect the ground to both the focus and fire wires to shoot, on some you only need to connect the ground to the fire wire to shoot. It is fairly easy to use a metal wire or even screwdriver to determine which method your camera uses (my Olympus cameras need both connected).
The basic idea is to have one pin on your Arduino that connects the ground to the focus wire, and another that connects the ground to the fire wire. I use an opto-coupler so that the Arduino is electrically separate from the camera (the camera provides a weak current through the wires, and when the circuit is completed, it does the action). You probably could use transistors instead of opto-coupler, but I started with an opto-coupler, and I prefer the extra layer of isolation (an opto-coupler has a led inside with a photo trigger, so the Arduino and camera never have wires connected together). Recently, I've been using CNY74-2 optocouplers with 220ohm pull-down resistors, but you might want to use this unit which is more self contained: http://www.ebay.com/itm/300723438237?ssPageName=STRK:MESINDXX:IT&_trksid=p3984.m1436.l2649.
On the left side, you would connect a 5v wire from the Arduino to VCC, ground to the ground connection, and the 2 pins that will control the focus and fire to CH1/CH2. On the right side, you would connect the ground wire to both - terminals, and connect the focus to the CH1 + terminal, and fire to the CH2 + terminal. You would then need to write a program that uses digitalWrite on the 2 pins to control the focus/fire buttons.
If you live in the USA, and still have a Radio Shack nearby, they have 2.5mm terminals, that you plug a cable into and it has 3 connections to solder wires to: RadioShack.com Official Site - America's Technology Store.
Typically you need to go to the back of the store to find the gray cabinets of doom, to search through for the connector. If you are lucky, the clerk can help with the search (many RS clerks have no knowledge of these type of electronics). Initially you can just twist the wires instead of soldering. Or you can just cut up a cable.
You need a tool like a multimeter (or a simple connectivity circuit that has a led with a resistor connected to a battery, and the led lights up when the circuit is complete -- you use this to identify which wire is which).