Consider starting on an electronic keyboard. Most pop songs use a variety of keyboard sounds, so later on, when you're ready to play with other musicians, you'll already have that under your belt. Since an electronic keyboard is set up like a piano, you will be able to play on an acoustic piano as well. If you use an electronic keyboard, consider buying a full-size keyboard that's touch sensitive. Touch sensitivity will allow you to play varying degrees of sound volume, just like you can do on an acoustic piano.
Use a beginner's piano chord book, which you can purchase at a music store or find online free of charge (see Resources). Chords are the backbone of pop piano playing, and the more chords you know, the more songs you will be able to play. Having a memorized database of chords in your head will also allow you to play along with other musicians on the spur of the moment (improvise). Chords on the piano are fairly easy to learn. Once you know the shapes of the different chord types (where your fingers are placed), you'll find that you can make chords in any key using the same pattern. The chord shapes stay the same, but the names change depending upon which note your first finger falls on.
Learn to read simple music notation. Use a music chart showing the notes on the music staff as they relate to the notes on the piano. You can find one of these charts online for free (see Resources). Consider buying a note chart the length of your piano that sits right in front of you at all times so you will always have a visual reference of the notes. You can purchase one of these charts at a music store. This basic note-reading ability is enough to get you started. You're learning pop piano, so the focus is on fun, not on music theory.
Begin playing simple music as soon as you can. You can purchase beginning piano sheet music at just about any local music store or even find simple sheets online (see Resources). Most music publishers publish simplified piano arrangements of popular pop songs.