Tbh i skipped half of the posts on here, due to it being people squabling and not actually addressing your question. I am 5ft 10 and jus over 10st so it is likely that I have pretty much the same figure as you (although i would love to have thinner legs). I to have a round face, and i always have had. I have played netball for 11yrs but this has not in the slightest affected the shape of my face. My advise to you, would be that if you want to take up exercise, good on you, but regulate it, as if you over-do it you could be doing more harm than good to your body. This regulated exercise should also help to tone your stomach. In reply to what some others have said, abdominal crunches/sit ups can be good for toning, but if not done properly they can cause serious problems for your lower back, so be careful if you are to include them in your exercise.
Hope this has helped. x