Lean Cuisine and other pre-packaged food are a good place to start, but I find I can eat a lot more for my calories with home made foods. I eat the same thing all week, which might not work for you, but it simplifies my cooking to one day a week.
I also focus on whole foods and lots of vegetables, because it is important to me to be full. I can eat lots of broccoli for 100 calories.
My basic menu:
Breakfast

atmeal with cinnamon, almonds, rasins and nutmeg, made with water and brown sugar. You can adjust this to the number of calories you want.
Lunch: Hard boiled egg, broccoli, a grain (quinoa this week) and fruit. Nonfat greek yogurt for snack. (120 calories, 16 g protein)
Dinner: Huge salad, with cottage cheese and avocado for dressing, and another protein source.
I am eating about 1300 calories a day. I eat mostly vegetarian, so I focus on protein.
Good luck!
