At 12 weeks, there is not much. He is still so small. Just make sure you pack everything they need to care for him. Make sure you have picked a great provider and that you trust the person who will be in charge of him. Other than that, not a lot at his age.
as for the breastfeeding, make sure he is not just being fed from the breat at home but also sometimes a bottle of pumped milk. That way the bottel will ot be so foriegn to him at daycare./ other than that not much, just ride it out and trust your provider.:o)