Unity is more beginner-like than UDK (by far, in my opinion) and gives you a cleaner way to get code into your game.
All you need is to get good with C# and programming in general, and learn the Unity classes (GameObject and Transform are important). All you've gotta do is write a script and drop it on something in the game to get the code running.
I would suggest using Visual Studio C# Express or MonoDevelop to write your code. They have convenient features that will help you in the future.
Bookmark/favorite these pages, they're very useful to have around for when you need them:
- This one for the search bar; it lets you search any class in Unity and see all of the methods and variables inside it
- This one to find information about specific components and how to use them (such as colliders, rigidbodies, lights, etc.)
- This one for whenever you need to configure your colliders/rigidbodies; it's helpful because at the bottom of the page it gives you a chart of which kinds of colliders will collide with others
- This one for all you need to know about C# syntax
Also, since I believe you're programming a 3D game, you should know how Vector3's work if you don't already.