Yes you should! You should start taking more vitamins and eating healthier, especially if you're the kind of woman who loves to eat junk food, fast food, drink alcohol and smokes. A doctor or nutritionist would be the best person to tell you exactly what you should be taking in order to begin this process.
If you eat healthy now, you needn't change too much when you do get pregnant. Ensure you are getting enough of the right nutrients every day. Moms to be sometimes are sure to eat a snack or two each day that includes nuts, beans, fruit and vegetables, on top of their regular meals. You may find you have cravings for some foods and that your intake increases, however. Follow your body's cravings as long as they are safe.
If your current diet consists of junk and fried foods and even a lot of alcohol, then yes you should change your diet to a healthier one. During pregnancy, it is important to create a healthy environment for your baby to grow, and it will need the best nutrients to develop well. A suggestion is to go on a 'clean diet' where only healthy foods are consumed such as vegetables, clean proteins and carbs with a low glycemic index.
As best as I know, you will need to increase your calcium intake. There are also pre-natal vitamins on the market that can help with increasing the necessary vitamins and minerals. However, like the others who posted before me, I agree that following the advice of your doctor is the most advisable.
Obviously, your doctor would be the best person to answer this question, but I think there is some preliminary things you can do before an appointment. It is usually recommended that you eliminate caffeine from your diet and if you are on the heavier side, to lose some weight to get down to a healthy point so it’s not so hard on your body when you do get pregnant.