Make it something you enjoy!
Walking, jogging, aquaerobics, swimming, Zumba, ballet, biking, tennis..
Walking is a great exercise. Start slowly and build up until you reach the optimal daily level, just put on a pair of comfortable shoes or trainers and off you go.
You don't have to do 30 minutes at once if you don't have time, you could break it up into 10 or 15 minute sessions. A brisk walk for example would be covering a mile in around 16 minutes.
Just remember, ANY MOVEMENT COUNTS!
Walk up the escalators, take the stairs instead of the lift, park further from the shops, be brisk in Brent Cross! Even hide the TV remote. Just move as much as possible.
A Pedometer is a great way to monitor your steps. Set yourself a goal, for example, aim to build up to 10,000 steps a day and you'll see and feel the difference.
Most importantly, be active and enjoy, and make sure you take a rest when your body tells you.