Why did you want to get Realm First Level 90?
Why? Mainly because I had never done it before and lots of people in my guild were getting really excited about Mists and all the hype... It was hard not to get all wrapped up in the excitement! Besides, it was something I had always wanted to do and just wanted to see if I could do it. And I'll show a little bit of hubris here: its always nice to see your name go across the screen in realm spam.
What did you do to prepare in advance?
Well, I had leveled to 90 on the beta. That said, I did that leveling very early on in beta so I knew there was going to be a number of changes (as well as bug fixes) that i didn't experience. I was familiar with all my class changes, so I knew I was going to be comfortable with both healing and DPSing in Shadowform.
Perhaps the two best things we did to prepare was to formulate a strict strategy with our 5 man team and to plan our dungeon time. Our 5 man leveling team had a rogue, Torima, who had quested a lot on beta and has a phenomenal memory so he somewhat became our de facto guide through the maps and quest chains -- he actually remembered some quests that were dead ends or particularly painful in getting RNG quest drops: if the quest was skippable, and ate up too much time we didn't do it -- such as the Yak quest out in Dread Wastes.