yep, what RicF says, plus you could go walk, then brisk walk, then slow jog. don't over do it, and use one of the walk/run programmes of the Couch to 5k. 3 times a week fine to start with (or indeed forever). once you start quite enjoying it (which you will!) then go to a running shop and get fitted for trainers - bad-fitting shoes much more likely to harm you than lack of a warm-up.