On its first couple of moves, the computer will typically put its first O in the center space (as usual) and its second O in the lower middle space once you place two Xs in diagonal corners. Therefore, spend this turn placing your X in the top middle space to block the computer's win.

In the 1970s MIT students built an unbeatable tic tac toe computer out of tinkertoys. No. It's called impossible for a reason. The closest you can get to beating it is a draw, it's programmed to be impossible.