You can start by making substitutions in your diet, replacing less nutritious foods with more nutritious foods, for example in place of potatoes, rice or pasta, you could have broccoli, peas or carrots. This will both decrease your calorie intake, since you're substituting dense high-calorie foods for bulky low-calorie foods, while also increasing your nutrient intake. If you want to get into it in more detail, you could add micro-nutrient supplementation by taking a broad spectrum vitamin and mineral pill each day, and if you want to get into it even more, you can replace low quality protein sources like beans with high quality protein sources like eggs, which your body can utilise a higher proportion of, allowing you to reduce your total protein intake without ill effects.
You can also plan your meals and supplements to maximise your absorption of nutrients, so vinegar can be added to meals or you can take vitamin C with meals to increase the availability of minerals in the food, and foods or supplements high in calcium can be taken at a different time from foods or supplements high in zinc so that the calcium doesn't interfere with zinc absorption and so on, since each nutrient often competes with several others for the same uptake sites.