If you don't have a routine already, do Starting Strength 5x5. Builds muscle quick, gets you strong quick, and lays a foundation for later down the road
To put on weight...chicken, wheat pasta, brown rice, and peanut butter are your best friends
Here are foods I typically eat for each meal
Breakfast - steel cut oatmeal, eggs, peanut butter toast, protein/granola bar
Snack - nuts, trail mix, nature valley bar, piece of fruit
lunch/dinner - a protein (chicken, fish, steak, etc..) + a carb (rice, pasta) and frozen veggies(brocolli, corn, etc)
The key is not just to eat large meals, but to eat often. Eat 5-6 meals a day. 3 large, 2 snacks