Get these accessories if you don't already have them:
bike shorts - more expensive shorts are worth every penny, better padding will make a huge difference (no need to wear underwear under your shorts, go commando) (it's normal to get sores, any medicated ointment will help. there are specialized bike creams: gesass cream and chamois butter)
seatbag - hex wrench for quick adjustments, spare tube (take the spare tube out of the box and put it in a plastic bag, the box takes up too much room and the bag prevents holes from being worn into the tube), 2 CO2 cartridges with a CO2 inflator adapter thing), 2 or 3 tire levers
2 cages and 2 water bottles (at least)
bike computer (this will keep track of your speed, mileage, time, etc)
sun glasses
helmet (more expensive helmets are lighter, really the difference is nominal. anything over $35 should be fine. Bell and Gyro are the best brands).
bike jersey with pockets in back
power food - powerbars, cliff shots, cliff bars, powergel - i liked to go cheap and healthy with dried fruit or raisins and nuts
bike gloves (optional, i never wore them, if your hands get sore in your training buy some)
bike shoes and pedals, clipless, being able to pull up with each stroke as well as push down makes a world of difference
tri-bars - not recommended for a beginner, but maybe later down the road
mirror (helmet or handlebar) - optional (I never used one)
play with your handlebar and seat position while you train to find something that's comfortable. if your neck or back get too sore you may want to have those adjusted. it's an easy adjustment to do yourself - otherwise most bike shops will do it for free. Seat can go up/down, back/forth and tilt at different angles.
likewise play with the adjustments for your pedals to find an appropriate tightness. If you're new to clipless pedals you don't want to get stuck and fallover (it happens), but to loose and you'll fall out when you don't want to. Again most bike shops will help you here for free. learn how to change a tire. it's really easy.
put sunscreen on before you head out. Really get your thighs and shins good - they will get a lot of exposure! Also your face and arms and hands. And don't forget your forehead. Bike helmets have holes in them and you'll end up with a hilarious forehead burn if you don't sunscreen it.
Try to find a speed you can maintain for the duration of your training session. Don't kill yourself going fast for a short while just to slack later.
Never set your gears so that one is in the highest setting and the other in the lowest setting. It can cause the chain to fall off since it's being twisted.
When going up a hill don't stand and pedal unless you absolutely have to. It kills your speed and efficiency.
Always bike with traffic. Use hand signals. When biking in a group there is certain courtesies to follow regarding announcing traffic that you see (car left, car up, car back) just to give other riders a heads up.
Drafting (following closely behind another rider) makes a HUGE differenceto your speed and energy expelled.