I get asked that question all the time. I am a jazz piano teacher in Los Angeles,and I'll give you some tips to get you started.
First, you need to have some technical foundation at the piano, and nothing beats classical for building basic skills at the piano.
Second, you should know that jazz relies very heavily on your ability to hear something, and then play it back on the piano with the same style and correct notes that you heard. That is called ear training, and you can start doing that right away on your own.
Third, there is nothing better than finding a good teacher in your area to help you out-online learning only takes you so far, and sadly there is a lot of bad material online that a teacher can help you weed through.
Fourth, you can take a look at my book series on jazz piano playing-three books that will build your foundation as a jazz pianist and musician. You can read about the books and hear one of my etudes at http://www.planetmullins.com/book.htm.
The etude book has become very popular with classical teachers here in LA learning to play jazz.
Best of luck!
Rob Mullins www.planetmullins.com