When are you planning on doing this race? If you've never done anything like this before and your fitness level is poor, then you need to spend at least 6 months prior to the race training.
If you can, buy a Polar Heartrate Strap, so you can monitor your heart rate. A good way to judge if your fitness level is improving, will be when your heartrate doesn't rise to the same level as when you started.
Start off walking at a moderate pace for the first week, then build up to a brisk walking pace in week 2-3. When you feel you are ready build up to a jog, i.e walk for 3 minutes, then jog for 1 minute, then back to walking, keep repeating this.
Over the weeks, try to increase the length of time you are jogging, but do this gradually, eventually increasing your speed and pace. You should also aim to increase your distance over the months too, so eventually you will complete 5kms, but make sure this is not done too close to the race day.
Ensure you wear appropriate running shoes and keep hydrated, if you do not, you are more likely to suffer cramps and shin splints. Also ensure you warm up first, to prevent muscle tears and ideally stretch after your session, to maintain flexibility.