Spring seedings are always a challenge, and it's best, at this point, to simply stay the course and let things grow in as well as they can. Feeding right now is not necessary, and it's actually a really bad idea on a new lawn. Hold off until late May and let things grow in and stabilize first.

At that point, you can use a starter if you really want to--once--or just use a normal lawn food. Either one can go at bag rate.

We really need more information.

Where do you live? If you live in the south you'll get one answer. If you live in the north it's different.

What kind of grass did you seed? If you seeded bermuda in the south, it will come in weak until June. If you seeded fescue in the north, it's going to be challenged by the increasing heat of late spring and summer.