After not having owned a bike since I was 17 (I'm 30 now) and setting some goals around losing weight and fitness, my wife and myself have ordered a couple of bikes from our LBS (see I've started reading the forums and picking up on some acronyms

After some discussion, the guy in the LBS suggested a Giant Boulder Disc for me, and a Giant Cypress for the trouble and strife which fit to our budget. Short term I'm looking to ride the bike to the gym (2km) and go for rides down to Jells park on the weekends (I'm in Glen Waverley). I'm then intending to start commuting to work (23km) taking the train at first part of the way in, then riding the rest. Ultimately I've set a goal to participate in the BRW triathlon in 2009. Why so far into the future? Well I weigh 150kg. I need to lose a truckload of weight before I can run - else I risk some joint damage. Doing the math at a healthy 1kg/pw I won't have close to an ideal BMI until April 2008.
Having talked to a friend who does a bit of riding and the guy in the LBS, this is a little more detail of what I've ordered.
17" Giant Boulder Disc, semi-slick tyres (as I expect to spend most of my time on tarmac), a helmet (entry level Netti), a bike lock, a kit with puncture repair, tools, pump and pouch that goes under the seat, lights, drink bottle/holder and a basic computer (speed, distance etc).
Anyway, enough background. Onto my questions.
When I rode, it was in a small country town - population 2000, I now live in Melbourne, and obviously traffic and rider density is much higher - I'm not sure of the best ways to deal with traffic when I'm not on bike tracks, apart from my perspective coming as a driver. Are there any unwritten rules that apply when dealing with cars/other riders? What should I consider as I start riding in city traffic?
My bike/accessories - I think I've got all the basics - is there anything else I should be thinking about?
I'll continue reading the forums as there appears to be a wealth of information in here, it just takes time to read through everything, and I want to minimise the amount of time I spend in front of a computer which led to my getting seriously obese in the first place.

I thank everyone that cares to comment in advance - I really appreciate your insight in this area that I'm rediscovering.
Cheers
Pugsly