Minimax Evaluation Function

I implmented Tic-Tac-Toe using the Minimax algorithm (yes, I realize that I
can use Negamax, alpha-beta, etc. for improvements, but for the time being
I'd like to keep everything as simple as possible).

I have everything coded up but I am running into a problem because I am not
sure how the Evaluation function is supposed to score a given position. I
mean, I understand how to code the heuristic, but I don't know *which* score
to apply to a given node.

For example, say a given position is a win for X, do I have my Evaluation
function *always* return +Infinity for that position, or do I have to take
the *current* player into account, etc.?

I hope this make sense... I can post code if my question isn't clear.

Thanks!
BT