I would wait until fall or later in the spring to fertilize to let the new turf get a good hold without possibly burning the tender roots of the newly growing grass. In fall or late spring, use a fertilizer that is higher in nitrogen (the first number in the N-P-K number). This article will help: https://www.gardeningknowhow.com/lawn-care/specific/st-augustine-grass/st-augustine-grass.htm