What is the best type of grass seed to use in Florida?

I live in florida, and my back yard looks horrible. Once it starts getting a little warmer outside i would like to buy a few bags of grass seed for my back yard. Any suggestions to the type of grass seed to use and about how long does it normally take to grow.
FYI: i would like something a little thicker then rye grass, and something that will last for a long time and work well with florida weather.
Thanks in advance