Diet for breastfeeding?

Hi all, I'm due with my first baby next Friday and I want to try to exclusively breastfeed for as long as possible. I'm open to formula too if needs be. I'm just wondering (as I'm being fed all sorts of information) how important it really is to keep a strict, healthy diet when breastfeeding? As I've been told that EVERYTHING I consume will pass through my milk and onto the baby. I don't necessarily eat badly, but I certainly don't eat super healthy either.