Trekw17, you can learn to play the piano and use that knowledge to later learn to play the organ, or transfer that knowledge to the organ, but you really need an organ to learn how to play one. If I wanted to play the organ and didn't have one or access to one, I would learn to play the piano until I had access to an organ. That way you would be miles ahead.
The second part of your question is more difficult. People have varying degrees of "ear ability". There are a few who can play anything easily by just listening. The rest of us have to study hard to develop our ear. You have to learn chords and how to access them immediately. You have to listen to a lot of music and see how it flows. You have to learn progressions. You have to play along with the music you are listening to and learn to recognize keys and be able to play within them. As you do this, your ear will develop. It also helps to start playing with other musicians and to try to play along with a choir or a singer as much as possible.(as soon as you are able to)
There are many advertised CD and digital courses that are available to help you with this. Some of them are very good.(some, not so good). Hearandplay, Giftedplayer, Gospelkeyboard are some of the better ones. There are others.
T-Block has posted enough material that a person could use to learn to play by ear in this forum at no cost. Just do a search for his material. He has a list posted showing the material that he has made available. There are others who have posted excellent material in this forum also.
