Turn off your computer and/or television and go outside and do something. Take a walk, ride a bicycle, mow the yard. Find a park that has a parcourse (check spelling), it is a cross country style course you can walk, jog, or bike that has small stations spaced out along it that you stop at and do a different exercise/activity.
There are a number of things that you can do in order to be active. Some of them include running, riding a bicycle or doing any sort of sport. Swimming is also a great way of being active. You need to avoid sitting in one place or watching TV.