well, there is acoustic piano, and there are keyboards.
the principles of playing are the same, but the sound and touch are different.
the piano is a very complex instrument capable of very beautiful noise, and if you are serious about it and are willing to practice at least 1 hour a day every day you can be reasonably competent in maybe 5 years.
by that i mean you should be able to pickup any piece of sheet music and sit down at any keyboard or piano and play it.
i have played acoustic piano for 40 years, and taught myself to play.
first, learn to read music, both bass and treble clefs---so you know where the keys are for the particular notes you want to play.
buy yourself an old upright piano for a few hundred bucks---get it tuned---use a felt tip sharpie pen and write the note names on the white keys---write the corresponding note names on your sheet music. (by the time you know the notes you will have worn off the magic marker on the keys)
go buy some sheet music of some songs you enjoy---if you don't enjoy the tune practice sessions will be tedious---don't practice playing "mary had a little lamb"---practice with diana krall tunes, or luther vandross, etc. ex: the tune "Dance with my father" is fairly simple to play but very beautiful, and Luther's sheet music sounds just the same as it sounds on the CD---and in 2 weeks you can play it if you follow my advice, and wow your friends---this will inspire you to keep working at pro level pieces.
for example, if you like whitney houston get whitney houston sheet music---there are some free sheet music trading clubs on the net---
http://groups.yahoo.com/group/PianoFiles/
join it if you haven't already done so.
some artists have fairly uncomplicated music to play that sounds nice (billy joel, paul simon, james taylor, carole king, burt bacharach, jobim) and some artists have complicated musical arrangements that are more difficult for newbies to play (mariah carey, tori amos, some norah jones, rickie lee jones jazz stuff, some nat king cole, etc) so stick with the simpler, but still pro level stuff to start---this will be more satisfying for you.
but if you want to advance fast and learn to play many different songs you must learn to read music.
get going and have fun.
(nothing sounds better than a big grand piano. nothing.)