You don't need to toughen up your nipples. Make sure you get help at the hospital to make sure your baby is latched right. It might hurt in the start while you both are figuring it out but once you have the latch down it should not hurt AT ALL. My advice is don't give up. The first 6 weeks are the hardest after that its a breeze.
Yea sunlight is the best option. Don't rub them too much or they will become irritated. It will come naturally when baby is born. A baby sucking is like nothing else. Haha
Direct sunlight toughens up the nipples too. Exposing them for 5min everyday will help. Good luck