Tic tac toe is perhaps the most complex game to have ever been played. You have to manage your points productively to put three of your units in a row. It sounds mentally exhausting, and it is. Don't worry though, just mindlessly put your marker down wherever you can block your opponent, and you'll lock him out of the win.
The last piece of advice here is the most important. The object of every game of tic tac toe is not to win. No, it is in fact to tie with your opponent as many times as you can. Whoever causes the most ties ends the game in a tie with the other player. Fortunately for you, 99.99% of tic tac toe games end in a tie, so you're sure to come in not last.