add fruit and veggies- fresh, steamed,
take out most carbs
Limit fruit to 3 servings/day because it tend to have a lot of sugar and if not used right away, turns to fat.
Try to center your meals around vegetables, because they have a lot of nutrients and fiber for very few calories. Also stick to low/no fat dairy, lean meats, good fats (such as avocado, olive oil, etc). Nuts and seeds are a good addition, because of the nutrients, but limit them because of the fat, even though it's "good" fat. Eggs are an excellent source of protein and vitamin B12.
If you buy packaged foods, the label should tell you how many servings are in a package and how many calories in each serving. If you are cooking fresh vegetables, etc, you can do an internet search to find the size of a serving, and the calories. For instance, if I type "calories in a serving of green beans" into my search engine, it will tell me that a 1 cup serving has 35 calories, or that Green Giant canned green beans have 20 calories for a 1/2 cup serving. You can do that for almost any food you are eating.