The best way to learn is to get lessons from a professional, however if you put in the time and effort you can learn some proficiency on your own - particularly if you only want to play for your own pleasure and maybe you have a few simple songs you want to be able to play.
I've never taken piano lessons, but I can play some simple things for the following reasons:
1. I understand the basics of music theory, and a little bit beyond the basics - I play flute and sing and my directors have always beeen big on theory.
2. My grandparents have keyboard instruments (grandma and grampa have an organ, grandma has a piano) and my parents and uncles' old lesson materials (like a sheet that fits across the keys to show which key is which note)
My dad bought me an electric keyboard for Christmas several years ago so that I could mess around. Its not as nice as a real piano but its less expensive and takes up less space.
3. I have a decent ear so if I hit a wrong note I can fix it.
One thing that I have done wrong is that instead of learning to play right, I just memorize the songs I enjoy.
As a side bar, my kids will be forced to take at least a few piano lessons because I very much regret that I was never given the opportunity - my dad likes to play but remembered hating it as a kid, and my mom doesn't even like to play (I know she was proficient at one time only because there are signatures from her instructor stating that she had passed each level of lessons), so they chose not to make me take lessons.
recent FOs: Shedir cap, Bricks of Atlantis, Pirates Hat, Citrine Socklets, Teddy bears, 1 entrelac sock
OTN: scraps blanket, Shedir for Teenie, Ice Serpent Hat
NIL: Qiviuk tam re-visited; weatherproof gloves; sweaters
My knitting blog: http://emilai333.wordpress.com/