There's a lot to learn about Unity, much of which can be accessed through our support page, but it can be a little daunting to know where to start.
I personally think that the two PDFs that come with Unity are a great place to start. They are: 1 - GUI Essentials.pdf and 2 - Scripting Tutorial.pdf and they can be found in the Tutorial folder in your Unity installation folder on your hard drive.
Experience programmers new to Unity will definitely want to check out our MonoBehaviour scripting reference which is a one stop cheat sheet for the base class every script derives from.
We also provide some introductory video documentation but there have been some really high quality videos popping up on the internet that I wanted to showcase.
Infinite Ammo's Alec Holowka of Paper Moon and Aquaria fame has recently been creating a series of Infinite Ammo Unity tutorials as well. I really like Alec's way of explaining not only how to use Unity but best practices in game programming. The picture in picture effect is also nice and adds a lot more personality to the whole viewing experience.