Check out the website - type in D23.com and it will automatically redirect you. Buy one of the "twenty three" magazines at Barnes and Noble, and also check out the upcoming events on the website.
If you like the magazine - you'll likely want the more expensive "gold" membership. It's $75 a year and will get you the chance to go to all events, plus send the magazine to your house. As the magazine is around $16 with tax, it's pretty reasonable to get a gold membership if you like the magazine (there are 4 issues a year).
If the magazine doesn't interest you, but the events on the
D23 website do, go for the $35 silver membership. I did this last year so that I could get Expo tickets (although I will be upgrading to gold this year)
And if neither the magazine nor the events interest you? Save your money for your next Disney trip