Getting started

Ride to work over the weekend before you start commuting to get an idea of time and distance
if you're running late, enjoy the ride - don't try to race the lights (it's not safe and you'll just end up stopping for another one further along)
don't make yourself miserable: work your way into cycle commuting a little at a time - there's no point in slaving through rain and snow when you're just getting started, and even one day a week of cycling reduces emissions by 20 per cent!

Bicycle equipment

An approved cycling helmet
A bicycle lock (invest in the highest quality lock you can)
Lights for riding in the dark, (you need these if you ride between half an hour before sunset, or half an hour after sunrise)
Front and rear brakes in working order
A bottle cage and water bottle; drink before getting thirsty to prevent dehydration (important for hot days, or long commutes)

The Highway Traffic Act (PDF) states that bicycles need:

A steady white light on the front of the bicycle and a red rear light or reflector if you ride between half an hour before sunset and half an hour after sunrise, and at any time when your bicycle is not discernible at 150 metres or less. A rear light is far more visible than a reflector, and can cost less than $10.
Rear brake capable of skidding the rear wheel on dry, level pavement. Obviously this minimum requirement is not adequate. The front brake of a bicycle does 70-80% of the braking work. The most effective way to brake is to use front and back brakes together.
A bell, gong or horn in good working order.
A strip of white reflective tape on the front forks and red reflective tape for the rear forks covering a surface of not less than 250 millimetres in length and 25 millimetres in width.

Helpful equipment

A bicycle map
Rain gear
Fenders to help keep you dry
Cycling gloves--even on warm days--they protect the hands if you fall
A flat fix kit and an extra inner tube
A bike rack, basket, or panniers to carry groceries, laptops, extra clothing, lunch
Mirrors (optional). Mirror use should never replace a shoulder check.

Sweat

If you don't have showers in your building maybe you can work out a joint-use agreement with a building that does
wait ten minutes after you arrive, then use a wash cloth and towel to refresh yourself in the washroom before changing into business attire.

Wrinkled clothes

Stock a locker with clothes for a few days
hang a garment bag on the back of the office door
use bicycle garment bag panniers.

Personal safety

Talk to a more experienced cyclist about safety and route planning
ride with a more experienced cyclist until you get the hang of things
read Toronto EMS tips on bicycle safety
take a CAN-BIKE course (416-338-0000).

Security

Lock your bicycle (both wheels) to an immovable object which cannot be easily cut or broken
use a good quality lock (a hardened steel U-lock, or a hardened steel chain and padlock)
consider using two different types of locks; that way a thief would have to carry two kinds of tools to steal your bicycle
padlocks should not hang low enough to be smashed against the ground with a hammer
never leave accessories like lights and bags unattended with your bicycle.

Take care
Although physical activity is almost always beneficial, you should gradually build up your level of exertion. Straining yourself may be counter-productive, or even dangerous. Men over 40 years of age, women over 50 years of age, or anyone who has a high risk of cardiovascular disease, or existing health problems, should consult their physician before embarking on any exercise program to which they are unaccustomed. Also, if you are very unaccustomed to physical activity, or if you are (or think you may be) pregnant, it's better to talk to your physician first. Finally, don't start on a new exercise regime while you have a cold or other illness. Getting healthier shouldn't be a risk.