http://www.cpsafety.com/ I like this site because it has good basic info, and photo galleries that show correct use, it also has a active forum so you can get your questions answered.
http://www.car-seat.org/ another great active forum
http://www.carseatsite.com/ this page has good info and alot of good links
http://www.safekids.org/in-your-area/car-seat-check-up-events/ find a child passenger safety tech. in your area to check your seats.
there are also a lot or youtube videos on correct car set installation and use and also some with cute pics and awesome facts two of my favorites are