I can not find how to find the biggest

First you need to get the average and then compare with each of the students. You can use an array. Try this way:

public static void main(String args[]) {
    Scanner sc = new Scanner(System.in);

    int n, cont = 0;
    double acum = 0, p_gen = 0;

    //IN
    System.out.println("Ingrese numero de estudiantes ");
    n = sc.nextInt();
    double [] promedios = new double[n];

    //PROCESS
    for (int i = 0; i < n; i++) {
        System.out.println("Ingrese promedio del estudiante " + (i + 1) + " : ");
        promedios[i] = sc.nextDouble();
        acum = acum + promedios[i];
    }
    p_gen = acum / n;
    for (int i = 0; i < n; i++) {
        if(promedios[i] > p_gen)
           cont ++;
    }

    System.out.println("El promedio general es: " + p_gen);
    System.out.print("Estudiantes superiores: " + cont);
    System.out.println("");

}

Create a variable called max which will take the value of the highest average that is fenced by entering, then show it by console.

Here I leave the code with the function to know which is the biggest:

   public static void main(String[] args) {
       Scanner sc = new Scanner(System.in);

    int n, cont = 0;
    double acum = 0, p_est, p_gen = 0;
    double max = 0;    
    //IN
    System.out.println("Ingrese numero de estudiantes ");
    n = sc.nextInt();

    //PROCESS
    for (int i = 1; i <= n; i++) {

        System.out.println("Ingrese promedio del estudiante " + i + " : ");
        p_est = sc.nextDouble();

        acum = acum + p_est;
        p_gen = acum / n;

        if (p_est > p_gen) {
            cont++;

        }
        if ( max<p_est){
            max = p_est;
        }

    }

    System.out.println("El promedio general es: " + p_gen);
    System.out.println("Estudiantes superiores: " + cont);
    System.out.println("Estudiante con promedio mas alto: " + max);
    System.out.println("");

  }
}

Create a variable that stores the average and then compare it with all students.

Example

public class PromedioEstudiantes
{
    private Scanner input = new Scanner(System.in);

    public void procesarCaso()
    {
        System.out.println("Ingrese el numero de estudiantes: ");
        int numeroEstudiantes = input.nextInt();

        double[] estudiantes = rellenarArreglo(numeroEstudiantes);
        double promedio = calcularPromedioEstudiantes(estudiantes);
        int estudiantesSuperiores = obtenerEstudiantesSuperiores(estudiantes, promedio);

        System.out.println("El promedio es: " + promedio);
        System.out.println("Hay " + estudiantesSuperiores + " superiores al promedio");
    }

    private double[] rellenarArreglo(int numeroEstudiantes)
    {
        double[] estudiantes = new double[numeroEstudiantes];
        for (int i = 0; i < numeroEstudiantes; i++)
        {
            System.out.println("Ingrese promedio del estudiante " + (i + 1) + ": ");
            estudiantes[i] = input.nextInt();
        }
        return estudiantes;
    }

    private int obtenerEstudiantesSuperiores(double[] estudiantes, double promedio)
    {
        int estudiantesSuperiores = 0;
        for (double estudiante : estudiantes)
        {
            if (estudiante > promedio) estudiantesSuperiores++;
        }
        return estudiantesSuperiores;
    }

    private double calcularPromedioEstudiantes(double[] estudiantes)
    {
        double sumaNotas = 0.0d;
        for (double estudiante : estudiantes)
        {
            sumaNotas += estudiante;
        }
        return sumaNotas / estudiantes.length;
    }

    public static void main(String[] args)
    {
        new PromedioEstudiantes().procesarCaso();
    }
}

If you need to know which students have what average, use a HashMap .