Connect 4 Artificial Intelligence

I am writing a connect four game in c++ but am having trouble adding depth to my current AI to add multiple levels of difficulty. I was hoping to add at least 2 more levels. I have seen a few other questions similar to this although I am a student and am having following the code in some cases. Does anyone know how I could approach this in a simple way or even leave a commented function? I am using eclipse on Mac but coding in the character set ASCII. Here is what I have at this point in time:

int summ :: AI()
{
string ax;
cout << "What's Your Name? ";
cin >> ax;

for (int a = 0; a <= 5; a++)
{
    for (int b = 0; b <= 6; b++)
        place[a][b] = ' ';
}

display();
int hold;
int hold2 = 0;
int charsPlaced = 0;
bool gamewon = false;
char player = 15;
string r;

while (!gamewon)
{
    if (hold2 != -1)
    {
        if (player == 15)
        {
            cout << ax << " what column would you like to drop in?";
            player = 254;
        }

        else
        {
            player = 15;
        }
    }

    while (true)
    {
        if (charsPlaced == 42)
            break;
        if (player == 15)
        {
            cin >> hold;
            hold--;
        }

        else
        {
            hold = (1 + (rand() % 7));
            srand(time(NULL));
            hold--;
        }

        if (hold <= 6 && hold >= 0)
        break;
        else cout << "nEnter a value between 1 and 7: ";

        if (cin.fail())
        {
            cin.clear();
            char c;
            cin >> c;
        }
    }

        if (charsPlaced == 42)
            break;
        hold2 = drop(hold, player);

        if (hold2 == -1)
            cout << "Column is fullnEnter another number between 1 and 7: ";

        else
        {
            gamewon = check(hold2, hold);
            charsPlaced++;
            system("CLS");
            display();
        }
}

    system("cls");
    if (charsPlaced == 42)
    {
        cout << "Draw!n";
        system ("pause");
        return 0;
    }

    if (player == 15)
    {
        cout << ax << " has won!" << endl;
    }
    else
    {
        cout << "AI" << " has won!" << endl;
    }

    system ("pause");
    return 0;
}