You can use a broadleaf weed killer like Weed B Gon to kill weeds. Read the directions to find out when it can be applied. Usually the new grass has to have been mowed a certain number of times, air temperature has to be in certain range, weeds need to be actively growing, etc. It is best to use separate products: weed killer for weeds, fertilizer for fertilizing. That way you can apply them at appropriate times. You can spot spray the weeds instead of applying it everywhere. If you have cool season grass, most fertilizer should be applied in the fall. Summer is not a good time. If you have warm season grass, summer is the time to fertilize. If your grass is stressed by heat, drought, or disease, hold off on herbicides until later.