Check the number of calories/serving on each package of food you eat, then take that times the number of servings you eat.
For instance, 1 medium banana (about 7" long) has 105 calories...
1 large boiled egg has 78 calories...
If you eat 1 medium banana and 1 boiled egg you eat 183 calories (105 + 78 = 183; if you eat 1 banana and 2 eggs, you eat 261 calories (105 + 78 + 78 = 261).
If the food you're eating doesn't have calories/serving listed, as most fresh foods don't, you can look up online to find the calories/serving.
You also have to include calories/serving of the things you drink or add to your food, such as sugar, honey, butter, etc.
There's a calorie counter app. You could try.
You can monitor the amount of calories in take on every meal you take, or you can try this application-- Calorie Counter - MyFitnessPal