How to learn the piano using my Mac?

Assuming you don't have a piano...

Using Garageband:

You could get a keyboard controller and plug it into the computer, and use GB's piano sounds. M-Audio makes decent keyboard controllers for the money. As someone with a piano background, you'd probably want to get one with a weighted keyboard.

Or, you could just get a digital piano with built in speakers, an all-in-one, and spend time with that. For digital pianos, I'd check out Yamaha and Roland. Incidentally, a digital piano could also be used to trigger virtual instruments in GB via MIDI.

Depends on how much you want to spend, really, how good a physical keyboard you want, and whether you want it to be self-contained, or trigger virtual instruments in the computer, and have the sound come out of speakers rather than the keyboard itself.

Then, I'd chose a song you like and try to learn it. It's always easier to learn something you like (and know). You might want to start with someone you played before, and re-learn it.

GB is fun because you can add extra parts and play around with a variety of sounds / virtual instruments. You could also use loops in GB to make instant accompaniments to play along with or improvise over.

You could also record yourself in GB and track your progress.

Finally, there are piano lessons you can download into GB (other instrument lessons as well). You might find them helpful.

"I definitely want a keyboard (at least 63 keys) that I can plug into either my tower or my MBP and do some kind of interface lessons through the Mac. Something where I can be taught the scales and basic songs again."

Here are some keyboard controllers listed on Sweetwater Sound's website (Sweetwater's a big retailer and their website is good for checking out gear):

http://www.sweetwater.com/shop/keyboards/keyboard-controllers/

Here is a link to their home pianos (they further subdivide into digital pianos and portable / arranger keyboards):

http://www.sweetwater.com/shop/keyboards/home-keyboards/

I guess you'd need to figure out how important it is for you to have weighted keys. Weighted keys will make a keyboard heavier and more expensive, but more satisfying to play. There's also a semi-weighted type of key (between a weighted keyboard and a synth action). The M-Audio Axiom, a keyboard controller, for example, has a semi-weighted action. It's a good controller, and some people love it. If you must have a weighted keyboard, however, you might not be satisfied with it.

It's hard to prescribe a keyboard type for someone else. Some people really don't like anything other than a fully weighted keyboard, other people like semi-weighted just fine. And for others, a synth action may be OK, although it doesn't sound like you'd be happy with a synth action if you are trying to regain any kind of chops.

For keyboards, Yamaha, Kurzweil, Roland, Korg, are good brands. For controllers, M-Audio is unrivaled at its price, although there are higher quality controllers if you are willing to pay for them. The brands already mentioned have some higher end controllers, and Nord recently came out with a (red) digital piano that's pretty good.

If there's a music store near you, you might want to go and just play a few different keyboards and see if there's a few that stand out. A lot of this is personal taste, influenced by time playing traditional pianos (and your fingers' expectations that they're thumping on a conventional mechanical action) compared to time spent playing synthesizers, so you might want to check out some of these keyboards in the flesh to see what feels best for you.

"Having never even opened up GB: does it have a series of lessons? Or is it more a recording studio."

GB is really more of a recording studio. There's some basic lessons you can download, and some "artists lessons," but it sounds like you'd spend more time working outside of those, and use GB mainly for its other features.

You're welcome. The 61es sounds like a good choice!