There are quite a few in-depth answers here, so I'll keep my answer fairly brief and based on my first experience of WoW.
I didn't even know what WoW was when I created an account. I knew absolutely nothing about the game other than it was a MMORPG and it had some blue thing with big ears on the front of the box.
No-one wants to read all the stuff about classes and factions at the beginning of any game, you just want to jump in and start playing. Unfortunately, knowing a little about each class is really helpful because your class determines your future game style. I just jumped in with a Alliance human Paladin, before re-rolling to an Alliance human Warlock. I found the warlock easy to play at the lower levels, where as I personally think that a Paladin took a bit more game knowledge.
I went running all over the place in my first couple of months, starting in Elwynn Forest and running into Westfall or Duskwood (even at level 7) and got completely owned several times. I didn't understand how gear worked or how to be a better player, this comes with experience. Either way, I enjoyed myself. I enjoyed the zones (even back then) and the stories. I especially liked Hogger (Elwynn Forest bad boy!) and having to get help to take him down. Even now at 85, I still go back to bash him about sometimes :)
I suppose the best policy is to just dive in, enjoy the game and not worry too much about the game mechanics or details, because you'll pick them up if you decide to continue to play after your trial.
Either way, enjoy WoW :)