Tic-Tac-Toe is a simplistic game with a very limited set of moves.
Starting out, one of the corner pieces gives you the best chance to win. Unless your opponent takes the middle with their first move, you win.
XO.
X..
...
X.O
...
..X
X..
..O
X..
I can draw out all the possibilities, if you need, but unless O goes in the center on their first move, X will win.