I wish someone had told me before I started cycling
Buy a rack and panniers. I wish I had discovered these earlier. Not only can you carry more stuff but they are a thousand times more comfortable than a hot, sweaty and heavy backpack. Get ones that snap on and off the rack, they're a cinch
Never change gears while standing up on the pedals. Sooner or later the chain will jump and you will pitch forward over the handlebars and kiss the pavement. As I was wearing gloves I managed to get away with only a grazed shoulder and six stitches in my chin.
Don't ride too close to parked cars. Sooner or later someone will open a car door which hurts horribly when you crash into it. Better to take up more roadway and suffer the occasional abuse from car drivers along the lines of "stay on the bloody footpath" then risk a catastrophic accident.
Get a bell and use it. When riding on footpaths pedestrians often behave like startled roos and have a habit of jumping into your path (usually in a very poor attempt to get out the way) Sadly, some people interpret the bell ringing as a demand to get out the way rather than a nice way of saying "here I come, please don't be afraid" You will probably cop some abuse along the lines of "stay off the bloody footpath".
If you are riding on the road in the wet get some clip in shoes and pedals. A foot slipping off a pedal at the wrong time could end very badly. Besides you'll ruin your regular shoes. Leave them under your desk in the office.
Spend some time working out your route to work. For most of us apart from the very fit and very brave, back roads, parks, cycle ways and even the footpath are preferable to competing with Sydney's crazed peak hour drivers on our crap roads.
Be seen. I shouldn't have to mention this but I still see (just barely) the suicidal or foolish getting about in stylish black without lights. Often at night. On a bike. I wear a flouro vest, plus the lights. Dorky or dead, your choice
Get a big bastard lock and leave it where you leave your bike for the day. Get something small and light and carry it on the bike for the occasional stop at the shops.
If you bought a mountain bike get rid of the knobby tyres. They make riding any sort of distance way too hard.
Learn to fix a puncture. Yes, it will happen.
< These are the Deuter panniers I've used for the last 7 years. I use both for the commute as having 2 balances the load and allows you to organise your stuff. Although they cost a little more than some the way they just click onto the pannier frame means they won't come off over big bumps.
< You're going to need a rack for those panniers. For commuting you only need something light.
< I prefer to use a bell as it doesn't cause the hostility yet get from using a horn. Some cyclists reckon an air horn is the way to go in traffic but I think it just makes drivers even more aggressive.
< These are what are called "dual platform" pedals. I use these as they give me a choice of wearing ordinary shoes or my cycle shoes. In wet weather I use my cycle shoes to prevent slipping off a pedal.
< This style of MTB shoe allows to you walk around on shiny floors or concrete. Useful too if you have to push your bike to a repair shop!
< Bike tail lights have improved enormously over the last couple of years. This one by Portland Designs is state of the art. This is something you should spend money on if you value your safety.