Now that your new lawn is up, you’ll want to feed it with a good lawn starter food.  The weeds will have to wait since weed killer can also kill very young grass.  Weed killing will have to wait until September.  Here’s a link to a blog post that talks about the starter food toward the bottom of the article:  https://blog.timesunion.com/gardening/starting-a-lawn-from-seed-its-time-to-get-crackin-2/7452/