You should aim to build up to 30 minutes of moderate intensity activity on 5 or more days of the week. If 30 minutes all at once seems too much to start with try:

•  Beginning with 5 minutes at least 3 times a day
  
•  Gradually build up the time until 30 minutes feels much easier.
  
•  Once 30 minutes feels easier try to work a little harder e.g. brisker walk, faster  swim.

As you become fitter you will be able to do more, but always remember regular moderate activity is better than occasional vigorous bursts.

Moderate intensity activity means, working hard enough to be breathing more heavily than normal, slightly warmer, be able to talk but not hold a full blown conversation.