Why should I do it?

• Cycling is fun (http://www.youtube.com/watch?v=SXApPXKzmDw) and good for you (check out these stats).
• You can be better off cycling than going by car: for example, going by car is more expensive than cycling -- the average price of a car is £16k, whereas bikes average at just £279, and changing to cycling could save hundreds of pounds a year just on petrol costs, leaving loads of money which can be spent on other things. (This site has a load of details on the comparison bikes and cars).
• And, of course, it's great for the environment: A typical family car will generate x amount of pollution per journey, enough over the course of a year to ... By changing just 1 in 4 journeys to cycling, you will reduce carbon emissions by x amount, meaning more squirrels will live and the sea won't flood your house. (Do the sums for yourself with Defra's carbon calculator).
  

How do I do it?

Preparation

• a bike -- obviously. If you don't have one already, you can get one comparatively cheaply from places like Halfords; otherwise, a friend or family member may be able to lend you one.
• some safety gear -- to be safe on the roads, you'll also need a helmet, some lights, a bike lock and some other stuff.
• some determination -- you'll have to want or decide to cycle, otherwise it's never going to happen.

Action

• Set yourself a target of number of times to cycle somewhere when you normally would have gone by car or bus, maybe just once a week for starters.
  
• Pick some routes with good cycle tracks, lighting and where you know your way around.
• Let someone know where you're going: not only is it safer, you'll also impress them with your healthy regime and can tell them about climate challenge.
• See if anyone else wants to get involved; maybe you and your friends could cycle to the cinema rather than nagging someone for a lift.
  

Rinse and repeat

You probably want to keep an eye on your progress, whether that's just proudly feeling your leg muscles every evening or keeping a detailed diary of routes, distances, times, weight etc.

And don't forget to keep going! No one-offs, please...

More info: