Welcome to the Dark Side...
My friends were all triathletes before I got into it so I was lucky to have coach/mentors from the get-go. I wasn't given any specific instructions at first, other than "ride more, swim more, run more" until I started getting serious with a season-long plan. I think almost anyone can finish a sprint tri, as long as they have some background in all three disciplines and aren't afraid of open water swimming. Oh, and a couple of bricks before your first race wouldn't hurt, either.
My tri club is also very active and offers tons of lectures, clinics and beginners swim/rides. Perhaps you can find a club in your area. I have found that club people are very helpful to newbies... I think it justifies their own obsession when someone else wants to try it! :D (You mentioned trying tri-training again so I'm not sure if you've raced at all in the past)
As for diet, I'm not very strict on what I eat, but have come to learn by trial and error what works for me and what doesn't. And, what works for me is not necessarily what works for my friends. One of them has still not forgiven me for taking him to an Indian restaurant the night before a 60-mile ride. (Visualize the Alaskan Pipeline. Now visualize it exploding.) :eek: ;) :rolleyes:
EDIT: Should clarify, when I say I didn't alter my diet much, I mean that I have always eaten pretty healthy. I kept it around 1500 calories a day + the # of calories I exercised that day. No fried foods, almost no red meat, fresh seafood, whole grain everything, egg whites, no trans fats, no cheese or butter, etc. Easy to do in SoCal, even when eating out (Sorry, K!) When I started training hard, I changed my carb/protein ratio but still kept it much the same.