Yes you need an instructor for a little while at least.
Gotta know how to move your fingers over the keys, there is a trick to this. Running your scales - going up scale so you do not run out of fingers - you only have 5 and the scale has 7 notes you tuck your thumb under grab another starting spot and keep going and coming back you reach over with the middle finger, etc. Did you know that? It's things like this the instructor will give you. And save you a lot of time. If a teacher is not in the cards do a Google on Yoke Wong or Pete Sears. Pete has a instructional video for less than $20 that is well worth your time. Yoke has several free instructional videos. Her DVD lessons are good, rather expensive however.
http://www.google.com/search?sourcei...f1734aaf8628c5 http://www.amazon.com/Piano-Lessons-...5266148&sr=8-2
Everyone seems to start with Alfred's # 1 instructional book. It is time well spent. Our Daughter was my instructor and got me started.
After you know your way around the keyboard - have your left hand playing with your right hand - you may want to look into chord piano. Here you play accompaniment - chords with both hands, Left hand does one pattern and the right does another pattern. My point in saying this is after you know your way around the keyboard you do not have to go the classical route, there are several other roads you can go down.
I use the keyboard to work out melodies and I primarily play chord piano. It is a great additional instrument. Here is where you can find more help.
http://www.pianoworld.com/forum/ Check out the Adult Beginners Forum section.
Have fun.