Find a teacher. Seriously, working with a knowledgeable teacher will help you learn what and what not to do. With some regret, however, that's not what I did.
I had been a musician, playing clarinet and sax, in high school and college so I knew how to read music and knew a bit of music theory. I decided one day that I wanted to learn. I bought an inexpensive Yamaha keyboard and a beginner level book and started working through it. It took me about a year but I was playing songs using both hands and with lots of mistakes.
At that point, I decided I liked it and wanted to continue so I bought a new upright piano and continued learning from other books and finding popular and jazz music I wanted to learn and just dove in. I'm probably a piano teacher's worst nightmare but I don't care. I love to play and I would love to get better but, at this point, I just can't devote the additional time at the keyboard needed to do that. And that's OK by me.
