After planting new grass you should wait until the grass has begun growing and has germinated before applying fertilizer to the new grass.
Most fertilizers can actually prevent germination of new grass seed so wait until the grass seed has germinated and has started growing before fertilizing or applying weed killer to the grass.
After the grass has begun growing and you've mowed it a few times then it would be okay to apply fertilizer to the grass to help maintain a healthy and green lawn.