AngryAnt now of Unity has some excellent plugins available for performing AI inside unity. You can find them here. There are also some extensions to it available from here at Arges Systems. Their libraries are based on Open Steer.

This will likely sound discouraging, but AI is a non-trivial topic. If you wish to start getting an idea of the depth, you may want to peruse the public section of AIGameDev as well as the many papers referenced on its wiki.

Having said that, it'll help if you have a good idea of what you want to do, and if you keep it as small as humanly possible. Doing AI is kind of like saying making a game - it's too broad.

Update: two recent resources linked on AIGameDev are Intel articles called "Designing Artificial Intelligence for Games". Here's Part I and Part II. Both seem rather general at first glance, but might be useful as an introduction.

RAIN{indie} is a, free, complete solution for adding pathfinding, movement, sensing, and behavior to any Unity project (3.5 and 4.0).

To download RAIN{indie}, just click the link and follow the 2 steps. RAIN{indie}: http://rivaltheory.com/downloadindie

We put together a compilation of tutorials and documents from the RAIN{indie} AI community that supplements our own support resources http://bit.ly/W47KSO

FAQs to learn more about the differences between RAIN{one} and RAIN{indie} as well as other answers http://support.rivaltheory.com/documentation/faq

Email me jester@rivaltheory.com if you have any questions.

Here's a pathfinding project for Unity
to make those AI's not to walk into walls and look stupid ; )
A* Pathfinding

there is a really good book in this topic that i recommend everyone to read. programming agmes AI by example this book has no C# code and uses C++ class templetes. you can use generics and easily convert code examples. also it uses boost library but you don't have to use it.