get a good understanding of nutrition, i.e. understand what simple carbs to do your body, get an idea of calorie dense vs. low-calorie foods that fill you up.
learn how to cook your own food so you can control exactly what you're putting into yourself
it's one thing to follow a diet plan, but it'll help a lot if you understand why a certain diet will 'work' (either lose fat, gain muscle, change body comp in some way) or why it won't.
once you have that figured out, start a program that strengthens your entire body, and also includes some sort of conditioning work. if you make your conditioning fun, then you're more likely to do it. if you used to play bball, even shooting some baskets for a half hour a few times a week will do something for you.
that's some pretty general advice, and i'm sure people could build on it. if you can, read michael pollan's "in defense of food"... it will help dispel a lot of the dietary b.s. that you may hear. otherwise, be consistent and work hard.