I would definitely start off with walking. Walking has been shown to be just as beneficial for health, fitness and weight loss as running, albeit it's not as intense.
I was around 20 stone (5' 9") at the start of the year, and started off walking a mile a day, and gradually increased it to two miles after a week or so.
I started going to a gym when I reached about 17 stone, and used the treadmill for walking for the first few weeks, as well as the cross-trainer and resistance machines.
I didn't begin running until I was about 16.5 stone, and even then I would get a sore ankle each time.
Now I am about 15 stone, and can run for about 20-30 minutes, however I still get a bit of ankle pain afterwards, although nowhere near as bad as it was, and I'm hoping that the lighter I get, the less discomfort from my joints.
Running is very intensive, so the concerns would be the impact it would have on your CV system if you've not done it before or for a while, and haven't worked up to it yet, as well as the real strains running while overweight will cause to your muscles and joints. Knee, ankle and hip injuries can be very common when running, and a bad injury can really set you back, not to mention muscle tears or ligament damage.
So, best idea would be to take it very easy, very slowly, curb your desire to run for the time being, and build up to running gradually, step by step increasing fitness and losing more weight first.