The best way to figure out if it's "worth it" is to look up the cost of tickets for the trips you plan to take that would fall within the window of your AP (which is 13 months if you are looking at buying one now in time for your Thanksgiving trip). Add up the cost of those tickets, including tax, and then compare that to the cost of an annual pass, including tax. There are other benefits to the AP (free parking, which is only a savings if staying off site, discounts on merchandise and some food, potential discounts on resort stays). But I would focus just on the cost of the tickets. Keep in mind an AP includes park hopping and Photopass downloads (making a separate Memory Maker purchase unnecessary). You can look up the cost of tickets and AP from Disney's website (or you could also choose to compare the cost of tickets from a discounted place like
Undercover Tourist or Parksavers).
In your example (using Disney's site, hopper, price based on adult ticket, non-Florida resident):
one 5-day hopper = $439.59
one 9-day hopper = $488.84
Total = $928.43
one Platinum Annual Pass = $797.69
So in this scenario, if you are going to do a 5-park-day trip and a 9-park-day trip within a 13 month period, then yes, the AP will save you money. If you wouldn't normally get hoppers or have kids involved, you can look up the different prices for non-hoppers and kids and do the same analysis. For the AP, there is only one price...kids and adults cost the same.
As for how to go about buying an AP, you have several options. You can purchase the AP online anytime. This will allow you to book FP now, but the clock will not start ticking on the pass until you activate it when you get there for your Thanksgiving trip. You could also purchase 5-day tickets for your upcoming trip in advance (from Disney or anyone else who is an authorized reseller), allowing you to book FP and then upgrade those tickets to annual passes while you are there. The price of a 5-day pass will be applied to your upgrade, so you should only owe the difference between the 5-day pass and the AP at the time you do the upgrade. Or you could wait until you get there and purchase APs then, but then you won't be able to book any FPs before your trip, since you wouldn't have any kind of passes or tickets attached to your
MDE account.