Hi, Matt. There are several different types of door seals available, most with a rubber or plastic sweep that seals the bottom of the door to the sill. You should be able to find a suitable one for your particular door at any home center.
If your sill has much rot, you might want to dig it out and fill the hole with automotive Bondo, sand flush and repaint. Also, check the general layout to be sure that every surface promotes the water flowing off the door and sill.