Lots of good training and nutrition info at
www.beginnertriathlete.com. Also lots of inspiring stories of beginners losing lots of weight and becoming competitive. You've got a huge head start in that you're young and very recently a competitive athlete, so you have a decent fitness base that you will tap into.
Don't try to run train like you swam in HS (lots of short and fast intervals) or you'll just have injury problems. Just run or run/walk slowly. Don't worry about run or bike speed. That will come quickly as your weight goes down.
I'm no expert on weight loss, but what works for me are small and frequent meals (my "in between meals" are usually just apples or bananas at work, but they keep me from being hungry and overeating at regular meals), and no soft drinks!
Good luck!
Brian