In the Retail game, leveling is meant to serve the point of acquainting you with the world, and your chosen spec. I wouldn’t worry so much about what to focus on, and just enjoy the game.
Upon hitting max level, a whole new world will open up involving proper speccing and things like that, all of which will take some time to research so that you can perform your role in a proper manner within group settings.
One thing you can do while leveling, however, is begin practicing good habits such as keybinding. If you’re not familiar with PC gaming, this might take some time, but is important to ensuring that your game play is smooth.