If you're brand new

Start here

If you're an adult sitting down at a keyboard for the first time (or the first time since a few unhappy childhood lessons), this is the order I'd read things in.

1. How to Learn Piano as an Adult: A Realistic 30-Day Plan: the big picture and a calendar.
2. Keyboard vs Piano for Adult Beginners: if you don't have an instrument yet.
3. How to Play Piano: Your First 5 Lessons: posture, finger numbers, first chords.
4. Piano Chord Chart for Beginners: keep it on the music stand.
5. A 15-Minute Piano Practice Routine: what to do every day.
6. 25 Easy Piano Songs for Beginners: once you know four chords.

After that, branch out: reading sheet music, playing by ear, the circle of fifths. And if you'd like a structured course, here's my honest comparison.