As to what kind of dirtbike to buy- There's a really good resource right on this forum that walks you thru the power versus weight issue (and 2 stroke versus 4)
http://dirtrider.net/Bikes_for_Gals.pdf
As to learning skills- There are dirtbike safety training schools offered all over the country. I took my 10 year old to one last summer. I didn't get her a bike yet, but I got myself one! Me first in this house. Ha.
-There are also all sorts of mx schools popping up everywhere once you get a bike.
Mechanical stuff to learn- You are already giving yourself a good learning foundation just by the reading you've done. Way ahead of me before I had already made up my mind to buy a bike.
On-line there are resources you can purchase for motorcycle maintenance... But, those require tools, tools, tools, and a garage!
Start shopping for bikes, and talk to salespeople. Ask questions. Keep reading this forum.
Check out the vocabulary section. If you can afford to, buy some dvd's on the subject.
Heck x-games was on for 12 years before I suddenly woke up and took an interest. It's all out there for the plucking.
Enjoy! Enjoy!