Take it very easy. Start by walking for 2 mins and running for 30s. Gradually increase how long you run for and decrease how long you walk for. Walk/run for about 15 mins to start with, and increase your total time out too. This is what Couch to 5K does for you.
As furball says, invest in high impact sports bra and proper running shoes. Find a proper running/sports shop - Up and Running are nationwide and they are quite good. If you are anywhere near Sheffield Accelerate in Attercliffe are amazing and in London I'd recommend Run and Become in Victoria or SheActive in Covent Garden. They should be able to sell you a high impact sports bra too, although I like the M&S ones (beware that their sizing seems to bear no relation to reality at all). For shoes, you need a good hour and a half to get properly fitted and be prepared to spend £70+.
Don't buy running shoes mail order, and don't buy general purpose trainers.
You can also get a digital watch where you can set it to bleep at intervals, to tell you to change running/walking.
And I can't run without my iPod. If I have music, then I can run for miles. Without it, my feet don't move at all.
You might want to discuss with your GP if you have health concerns. Mine wanted to keep and eye on me for a couple of weeks when I started running after my CS. I'm sure yours will be really chuffed that you want to do something to help yourself - and good for you too.
I find it helps if I run the same route every time (I go out 3-4 times a week) so I can "feel" it getting easier.
Despite my MN name, I had not run for 18 months until about a month ago, and could do with loosing a stone. I started again on New Year's Eve with 2min walk/1 min run. I started doing a 5Km route from home and can now do it without walking - will have to find a longer route. I do it when DH is putting DS to bed every other night (he goes out the other nights when it is my turn for bed time). It's a great feeling and I feel so much better. My clothes fit better too.
For interest and inspiration, pick up a copy of Runner's World (WH Smith, big supermarkets). It always a picture of someone scary-skinny in tiny shorts on the front, but the inside is quite interesting and inspirational, even for newbies.
Have a look at Rea.
Have a look too at Real Buzz website too - all about healthy lifestyle and incorporating sport into your life. There's a section on running and a section on women's fitness too. And a route planner, so you can measure how far your run is, and blogs and forums too. It was my forum of choice before I discovered MN when I was pg.
Real Buzz
Good luck and keep it up. The first 10 mins is hell, but it gets better I promise 
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