Seeding an exiting lawn.
Right now is the best time of year to prep and plant cool-season, tall fescue lawns.
Here is a page that will get you started: http://extension.umd.edu/hgic/plants/seedingsodding-lawns
You will note that there are links to other pages, such as HG102, Lawn Establishment, Renovation and Overseeding, that has all the needed steps in detail to get the best results, including how to read labels and find the best seed mixes. It's likely that you won't find the best mixes in the big box stores, but instead will need to look on-line or at specialty seed supply places like Newsom or The Turf Center.