How do I fix the random selection and apply a minimax algorithm for choice?

Question

for this TIC TAC TOE game, I'm new to coding so I'm not sure how can change the A.I to play more intelligently, I know it's with the use of a minimax algorithm and utilities but i'm unsure on how to implement that into the player.

"apply a minimax strategy to select the minimum possible utilities. Here are the endgame case utilities: X winning case (Human) : +1 O wining case (DeepBlue) : -1 Tie case : 0."

void PlayerDeepBlue() { // Robot playing
 int choice;

 srand(time(0));
 cout << "DeepBlue playing randomly..." << endl;
 do {
 choice = rand()%9 + 1; // random values between 1 and 9
 } while (State[choice] != EMPTY_MARK);

 State[choice] = O_MARK;
 PrintState();
}