It is ideally best if you've been taking supplements for at-least 6 months, because this allows your body to build up the vitamins needed for optimum conception health.
In particular, folic acid is very important to be taking. It helps reduce the incidence of Spina Bifida in your child. You can normally find a good range in your local chemist.
Instead of buying folic acid on its own, it would be better to buy a formulated pre-conception formula that has all the necessary vitamins for conception health, like Elevit for women, or Menevit for men.
Iron is a mineral that most women don't get enough of. Making sure you take enough iron will help prevent pre-term labor, low birth weight, and anemia.
Check out this short video on why it's important for you to start taking vitamins for pregnancy now, and how they help you:
Of course if you are a healthy eater, you may already have most of the nutrients you need, but it's better not to guess. You will be hopefully housing a baby and want to make sure that you have the best home possible for him or her.
In choosing your vitamin supplements, it might be best to speak with your local chemist or pharmacy to see what variety there is, and which one is best suited for you.