Hello and welcome!
I'd recommend seeing the Smithsonian, The White House, the Lincoln Memorial, the Air and Space Museum, and the National Natural History Museum. You can also check out the Washington DC Starter Kit that was posted here (
http://www.travelpod.com/forums/index.php?showtopic=11967) it has more information on places to visit.
I love the Air and Space Museum, it's just awesome and definitely a must see if your into that sort of thing.