Answer: Newly planted grass must be watered as needed to keep the soil evenly moist (damp like a wrung out sponge, not sopping wet/saturated and not dried out) while it becomes established. Keep in mind that grass roots reach down about six inches, so the soil needs to be moist that deep. To know if you need to water, dig into the soil with your finger. (There is no set schedule, it depends on the weather and on your soil type.)
In addition, regular mowing to a height of three inches should encourage it to fill in and shade out or crowd out weeds. You should mow often enought that you never cut off more than one third the grass height at a time.
New lawns can't withstand herbicide use such as "weed and feed" so you are somewhat limited for now on weed control. You can dig out perennial weeds by the roots or spot treat with herbicide containing glyphosate. Read and carefully follow all of the label directions.
If the lawn is very weedy and the grass stays sparse, you may have some extensive work to do including some reseeding or renovation. The best time of the year for major restoration work is late summer to early fall.
In the meantime, you should run some basic soil tests and check the fertility and also the soil pH. The test results will tell you if you need to fertilize and/or add lime to help the lawn grow better. Your local county extension should be able to help you with the testing and interpreting the results, identifying weeds, and recommending an ongoing lawn care program based on your soil and local weather.
Enjoy your new home and landscape!
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