Assuming you don’t like the moss, the first thing you’ll need to do is rake all the moss up and get rid of it.  We can assume, due to the presence of moss, that the soil has become too acidic for grass to thrive so you need to apply limestone to counteract it. So, the next step is to do a pH test to determine how acidic the soil is so you can determine how much limestone is needed.  More on how to do a pH test HERE.   The next step is to get some good grass seed down and start watering and watering and watering to get it up and growing.  This will be difficult during the long, hot, dry days of summer and would be best done in mid-August through mid-September.  In fact, you may want to delay the entire project until then and live with the moss until then. More on starting a lawn from seed HERE.