How to learn to play the organ
How I did it: I had played piano for nearly ten years (since age 10) when I began learning to play organ. I could use the organs in my school's music department although I taught myself. After a few weeks I bought an old electronic console organ, which allowed me to practice much more in a way to fit my own schedule. (Older electronic organs can be very inexpensive-mine was $200--unless they are certain models of Hammond. Plus, you can easily make the organ play very softly so as not to bother the neighbors). After that it's just a matter of getting some good instruction books and practicing! :-)
Lessons & tips: As a pianist learning organ, I found four things in particular took some getting used to. First, unlike with piano, you can sustain a note forever. But to do this you have to hold down the key in question. This leads to a very different fingering style from piano that will take a lot of work developing and getting used to. (You will have to forget a lot about fingering that you may have learned on piano.) Second is that you will be using your right foot to control the volume, not to sustain. On organ, volume doesn't depend at all on how firmly you press down the keys but instead how far down you are pressing the expression pedal. (Basically, the more you are pointing your toes as you are pressing this pedal, the louder you will be playing.) You will have to teach yourself that you are not sustaining with your foot. Third is the pedals generally. They add a whole level of complexity to playing organ that is not present with piano. You CAN play organ without using the pedals, especially at first, but do learn them as they make the organ sound so much prettier. Fourth is the whole idea of changing registrations and selecting different stops-and if you change registration as you are playing, once again, it adds a lot of complexity over and above piano.
Resources: But What Do I Do With My Feet?: The Pianist's Guide to the Organ, by Janette Fishell
Complete Organ Method: A Classic Text on Organ Technique, by John Stainer