Why do you want to eat 1000 calories?
What food you choose depends on goal
For example for exercise enhancement might want to eat a lot of carbohydrate
You will need to exercise if you want to gain weight while gaining muscle without gaining fat
Eat low fiber high carbohydrate foods made of simple carbs immediately before, during and after wxercise
Eat high fiber complex carbohydrate foods when the time is far away from exercise time
Eat 1.8 grams of protein per kilogram of body mass or "weight"
This might be 0.81 gram of protein per pound of body weight
Lift weights 2 to 7 times per week depending on how sore you get
Make sure to have vitamins and dietary elements / mineral supplements to fill in nutritional gaps in high calorie diet