Congratulations on choosing one of the most effective to change your fitness levels!
The 'rules' are really quite simple:
Choose how and when you want to walk. Do you want to set aside specific time or do you want to build it into your everyday life?
If you want to set aside a specific time check out your local facilities, parks and newspapers. Look for walking clubs, they are a great way to meet likeminded (and bodied) people. Can't find one? Grab a couple of friends and start and advertise your own!!!!!!! Get your local council, facility etc to sponsor your new club!
If you want to build it into your everyday life consider how you get from your home to wherever you go. Could you walk all or some of the route? Do you drive to work when the distance is less than 2 miles? Could you cycle or walk it? Do you get a bus? Could you get on a couple of stops later or get off a couple of stops early? Do you drive your kids to school? Could both you and they consider the benefits of walking it? We have walking buses - if you have kids and think this might just be possible I'll give you more detail on this.
As for how far, fast, long, often, etc There are simple guidelines here too! Broadly you should try to accumulate 30 or minutes per day of moderate exercise on most days of the week. Moderate means that the activity leaves you feeling a bit warm and slightly out of breath. You should be able to hold a conversation throughout and you don't need to be very hot and sweaty.
So you could either set aside half an hour a day to 'do' a walk, or you could accumulate 30 minutes throughout the day, as it's convenient to your everyday living. You could do a mix of both! Build up the time and/or distance you walk and don't worry about the speed you walk at. Concentrate on getting a bit hot and slightly out of breath - don't attempt the hot, sweaty and breathless bit, that's for sporting heroes (and many injuries)!!!
Get back if you want more info, motivating poeple to increase their daily activity levels is my job, so I'm stuffed full of useful (and boring!) exercise information.