Set your expectations realistically - if you haven't been exercising regularly then start slowly and only go a short distance on fairly flat terrain. Until you are confident of your ability, don't go too far from your starting point - this may mean walking round the block several times.
Decide how long you want to walk for rather than setting yourself a distance - speed and distance will come as you get fitter. Gradually increase the duration of your walks. Try to walk about three times a week.
Swing your arms as you walk. Stand straight, with your shoulders back and take long strides.