The best thing you can do if you are trying to get pregnant is to keep your weight at a healthy level and make sure you are eating adequate calories each day.
Having too much body fat and having too little body fat are equally damaging to fertility. It’s best to simply maintain a body weight that is healthy for your height and frame.
Try to eat a healthy diet with plenty of fruits and vegetables. Additionally, make sure to take a folic acid tablet each day, since women of child bearing age tend to be deficient in this mineral. You can begin taking a prenatal vitamin now, while you are trying to conceive.