Good question, sounds like your lawn is in pretty bad shape. So what I would do first is buy a bag or two of lawn fertilizer mixed with weed killer. Do not use ground up as that will result in damage to your whole lawn and new growth will be hard. There are many weed and feed brands out there, as long as it kills the weeds and fertilizes the remaining grass. Throw it on your lawn and let it sit for about two weeks, also make sure you water every now and then. Now you have prepared your lawn for new seed. Before you seed, aerate your lawn, this will allow the seeds to sit in the holes to germinate. Water LOT'S to ensure the seeds grown and and develop fully. Once your seeds have grown into full blades of grass and you are now mowing your lawn, you can add some more weed and feed they call it to help maintain the grass and keep the weeds away.
Here is an example of the weed and feed product.
This is just on of the many brands, and you can find it at you local hardware store. Good luck and enjoy your new lawn!