The training vault sticky should be your first stop.....and Mark Ripptoe's beginner program would really be ideal for someone like you, you've got lots of untapped potential and plenty of room to grow.
If you eat more calories than you burn and get better on big lifts like squats, deadlifts, bench presses, military presses, barbell rows, your body has no choice but to grow.
For eating, just eat, don't worry about a 'diet'......adding a gallon of whole milk a day is a great idea too.....in your case you could afford chocolate milk for the added cals.