How to implement Minimax Search to TicTacToe game

Question

this game is user plays against the computer, which should never lose.(i.e. the computer always wins or draws).and this game should use a single String to store the board moves.I have made the TicTacToe without the minimax tree. but i do want to learn how to implement the minimax tree inside my code.

I really appreciate the if anyone comes with the code for it.