oh wow.. I'm sorry for your loss. However, great job on your part to take the good from this. I mean you seeing that you need to do something about your problem, rather than falling apart and using the mourning of your friend as an excuse to be lazy and eat whatever you want.
Anyway, my first word of advice, and probably the most important thing for you to do, is to study nutrition. Read on forums, buy books, subscribe to magazines, etc... start living and breathing health.. fill your head with it.. learn the facts. Second thing is there is no "magic diet" .. everybody's body is different, and a lot of things that work for one person will not work another. IMHO opinion, you should go see a nutritionist and let the "expert" tell you what you should be doing.. at this point, it will be on of the smartest investments of your life. but if you can't afford that (start saving NOW if you cant), there is alot of good advice to be found on the internet.
For exercising, IMO the absolute best thing you can do is jog. And when your not jogging, power walk. You'll be suprised how quick it whips your body into shape.. although quickness isn't what you should be after here.. this is a long term project... a project that is never over.. so there is no rush. Also try swimming.. it is one of the few exercises that works every muscle in your body. The absolute best thing you can do is hire a personal trainer, and follow their advice 100%. That is thing most people don't understand about health.. as cliche as it sounds.. you can't cheat.. or you truly are "cheating yourself."
Some health tips... hmm... if you don't have an addicting personality, I would definitely recommend you start drinking two 5oz glasses of red wine per day since your 6'0 280. One with lunch, and one with dinner. You HAVE to drink it with food. The flavanoids and antioxidants work wonders for thining your blood and keeping arteries running smooth. Just ask the French... some of the worst diets in the world over there, yet they keep thin and live well above the average life expectancy for humans. If you don't like alcohol, try purple grape juice.. it has just about the same antioxidants as wine.. but it lacks the alcohol.. which is very good for your heart if drank in moderation.
That said, you honestly get a personal trainer and see a nutritionist.. its the absolute best thing you can do. I am a very health conscious person myself.. I honestly don't care to much about the look anymore.. my little boy is the most important thing in my life.. not my me trying to get chicks.. but I will admit that the look is a nice bonus . The absolute best part though is the peace of mind from knowing that your body is functioning properly.. and the self confidence it gives you.. it's like a high all on its own. Be relentless in your pursuit.