Eat nutritious food that gives your body the fuel to be able to grow it. Rest is up to genes. I couldn't get a proper beard before I was 31yo.
How to grow a beard?
You want to get vitamins like biotin, vitamin A, vitamin C but of course a healthy lifestyle is the key where you get all your daily vitamins preferably from food, but supplements can help if you find it hard to build a proper diet. Even vitamin D is linked to beard growth. You will also make sure you are having good testosterone levels, so sleep enough and exercise. Eggs, meats, fish, dairy, nuts and leafy greens (for example spinach, broccoli) should be a part of your diet