Like any change or achievement you want to obtain, start with a long-term goal (run a 5k) and set shorter-term goals to get there (walk 20 minutes a day).
I recommending walking/jogging/running. It is easy to begin, there is nothing to buy, and progress can be easily tracked. Start with whatever your body can handle, no matter how small (walking 5 minutes). Do it daily. Once that become comfortable, increase the time your are exercising by 5 minutes. Continue this pattern until you've reached your goal! The added benefit, is that by doing this you've created a habit of exercising.