Connect Four/Captain's Mistress/Four Up -- KI

Hello.
I'm new at programmin "games" or things like games^^
aktual I try to programm Connect Four (or how we say in Austria: four in a row)

The game works verry vell, it has a "nice" GUI and a few extras, but now I'm trying to make a KI (can I say make a KI? - sorry, I'm not that good at english, I'm only 17.years old -.-)
I'm programmin with VisualStudio2010 C# and the programm / game is a windows-form-app
-
my plan was / is, that the Pc gives the fields, which are the next free on the top of each row, a number( the number is like an index, for example, 255 is that the Pc will win if it place his/its "Block" there...)

for the cases (with its "Block" it can win or it can underbind my Victory (so if 3 are in a "row")) it works.
but how can // shoud i programm the KI where the first Block shoul be set, and the second, and so on...
??

please help me (: