Find a bike. Almost any bike will do, a half-dozen different kinds of bike will fit the bill. If you don't already have a bike, an old mountain bike, a well-worn 3-speed, a forgotten beach cruiser, or an old-school 10-speed will each make a fine commuter bike, given the proper refurbishing.
Pump up the tires, lube the chain, check the brakes… or if you're not mechanically inclined, take the bike down to your friendly local bike shop and have them give it the once-over. Tell them you're planning to start riding to work; some bike shops offer a commuter special to folks like you.
Buy a few necessary things for a preparedness kit, like a spare tube, a patch kit, a set of plastic tire levers, a simple pump, and a multi-tool. Consider investing in a cable lock or U-lock and a flashing red taillight.
Find a place for your new preparedness kit. Put everything in an under-the-seat bag or in a special compartment inside your backpack. Resist the urge to borrow things from it or to move it around too often; leave it all there until you need it.
Find your helmet. Wear it.
Find a Velcro strap or a metal clip to attach around the cuff of your right pant-leg so you don't get any clothing stuck in the chain or sprocket. In a pinch a rubber band will do, or you can be truly stylish and roll up your right pant leg.
Be aware of the weather. Dress accordingly and be ready for things to change. No special clothing is required; wear more-or-less what you would wear normally, given the forecast, with a little more wind protection since you will be going so fast. A compact rain jacket is a nice addition to your preparedness kit.
Put your hands on the bars, throw a leg over the bike, and PEDAL! You'll remember how.
Pedal in the general direction of your place of work; feel free to deviate at-will from your normal, well-worn, over-crowded car route as much as time and whimsy will allow!
Take your bike inside and stash it somewhere secure, or find a safe place to lock it up outside. Start work feeling happy and alive. Reflect positively on the fact that you're now a small part of the solution! Smile, 'cause when your work day is done you get to do it all over again!