I want to make an AI engine, but i dont know exactly how to begin. (and if there are people who want to join me building an AI engine, just pm me)

Well, first you must decide what exactly your AI engine is gonna do. AI is pretty large, you need to specialise, its impossible to do everything. You could do

-Pathfinding
-Decision making
-Planning
-Generic Behaviors
-Theoric Games
-Human-Machine Interaction
-etc

What kind of AI problem do you want your engine to solve?

After you choose that, then you can begin to choose how your engine will tackle that particular AI problem.

If the engine is going to be generic, you have to start
with defining the abstraction layer.
That is, if your engine is for tilebased maps, you
have to define te general data that such a map should contain.
If this is done right, your engine should be usable on most
tilebased maps.

In the other end, you could create something like prolog,
witch is sort of a AI programming language, that can be used
and utilitised from the actual game. The game enters the data,
and the rules that should be applied, and then the system does
all the calculations and delivers a nice result.

The most difficult fact is that most games represent their
world differently, and the AI must fit this representation,
so it have to be specialized.

If you manage to create generic modules, or a generic interface
to your engine, then the engine could be used many places, and
because of this be to greate use to developers all over the world.

Good luck!

Okay...
The engine will be generic, and i will use the prolog language...

Since I do not know what is your knowledge on that subject, here are my 2 cents.

I think it is better to focus on a problem to solve since all AI techniques do not process the same type of data and do not adress the same problems. There are quite a few good books on these subject. I would advise you to look up AI Agents in Virtual Reality (ISBN 0471127086). I grant you it is an old book (ed: 1996) but it shows an approach where the different AI techniques access a single homogenous World interface. The approach is easy enough to follow and shows how to implement a little game implementing different AI techniques within the same world.

I also would advise to start out with simple techniques which can easily be debugged and where a state can be recreated: State Machines and Rule Based systems come to mind here.

You also can have a look at existing AI engine to have a look at their architecture and compare with your own needs.
For a list, check out the excellent game AI page:
http://www.gameai.com/toolkits.html

Hope that helps.
Ghostly yours,
Red.

I'm sure you will find the AI in FEAR document, from one of the presentations at this years Game Developers Conference interesting.