If you get yourself fit and healthy prior to falling pregnant, it will be much easier to get back into shape afterwards and you will also feel more energetic during and after pregnancy.
If you are overweight and in poor health during pregnancy, you are at more risk of developing Gestational Diabetes, High Blood Pressure and Pre Eclampsia, which can be life threatening to you and your unborn child.
Start by eating healthily, exercising at least 3 times per week and stop drinking Alcohol. As Gossip said, eat fruit and veg also lean protein like fish and chicken.
When you do eventually become pregnant, you should avoid consuming Nuts, Undercooked meats and shellfish, mould ripened cheeses like Stilton and Blue cheeses. Consumption of Tuna should also be limited and avoid Liver, as it's high in Vitamin A and potentially harmful to the Embryo.