Basic Java control structure issue.

Question 1

1) I have an issue with the loop that I have set in order to read how many subjects the user wants to key in.

Already answered by @Nishan in a comment. Just replace for (int i=1;i >= subjects; i++) by for (int i=1;i <= subjects; i++).

2) When I tried to print "Grade = " + subjectGrade); my compiler says it hasn't been initialized. Same goes to the GradePoint and subjectCreditHour.

Already answered.

3) And I couldn't figure out how to get the program to calculate the Total Subject Grade Points, Total Credit Hours, and CGPA. Because depending on how many numbers of subjects the user wants, I can't figure out how to get the program to take in the user's input and sum them up together

You already are on the right way since you have the accumulators you need:

double subjectGradePoint = 0d;
double CGPA = 0d;
double totalSubjectGP = 0d;
int totalCreditHour = 0;

Whitin your loop and after nested if-else blocks, you need to update subjectGradePoint, totalSubjectGP and totalCreditHour variables in each iteration:

subjectGradePoint = GradePoint * CreditHour;
totalSubjectGP += subjectGradePoint;
totalCreditHour += CreditHour;

Finally, after your loop calculate CGPA:

CGPA = totalSubjectGP / totalCreditHour;

Question 2

Java mandates method variables to be initialized before use, just initialize String value and it shall work fine. Please refer to below code extract.

    String subjectName=null;
    int subjectCreditHour=0; 
    String subjectGrade=null;

Question 3

java variable must intialized before use

import java.util.Scanner;

/**
 *
* @author jerem_000
*/

public class JavaQuiz1 {

/**
 * @param args the command line arguments
 */
public static void main(String[] args) {
    // TODO code application logic here
    Scanner input = new Scanner (System.in);

    String name;
    int ID;
    int tel;
    String email;
    int subjects;

    String subjectName;
    int subjectCreditHour=0;
    int subjectMark;
    String subjectGrade="";
    double GradePoint=0;

    double subjectGradePoint=0;
    double CGPA;
    double totalSubjectGP;
    int totalCreditHour;

    System.out.print("Please input student's name : ");
    name = input.nextLine();
    System.out.print("Please input student's ID : ");
    ID = input.nextInt();
    System.out.print("Please input student's telephone number : ");
    tel = input.nextInt();
    System.out.print("Please input student's email : ");
    email = input.next();
    System.out.print("Please input number of subjects : ");
    subjects = input.nextInt();

    for (int i = 1; i >= subjects ; i++) {  //I'm having an issue with this loop 
    System.out.println("Subject " + i +  " : Please input the following"); //I placed    the        variable i there in order to make the program print something like "Subject 1 , Subject 2, Subject 3 etc". Depending on the user's number of subjects input
    System.out.print("Subject name : ");
    subjectName = input.next(); 
    System.out.print("Credit Hour : ");
    subjectCreditHour = input.nextInt();
    System.out.print("Mark : ");
    subjectMark = input.nextInt();

    if ( subjectMark >= 80 ) {
        subjectGrade = "A";
        GradePoint = 4.0;
    } else if (subjectMark < 80) {
        subjectGrade = "B+";
        GradePoint = 3.5;
    } else if (subjectMark < 70) {
        subjectGrade = "B";
        GradePoint = 3.0;
    } else if (subjectMark < 65) {
        subjectGrade = "C+";
        GradePoint = 2.5;
    } else if (subjectMark < 55) {
        subjectGrade = "C";
        GradePoint = 2.0;
    } else if (subjectMark < 50) {
        subjectGrade = "D";
        GradePoint = 1.0;
    } else  {
        subjectGrade = "F";
        GradePoint = 0.0;
    } 
}

    System.out.println("Grade = " + subjectGrade); 
    System.out.println("Subject Grade Point = " + (GradePoint * subjectCreditHour)); //I'm having a problem with the subjectGrade, GradePoint, and subjectCreditHour, it says variable might have not been initialized 

    System.out.println("Name : " + name);
    System.out.println("ID : " + ID);
    System.out.println("Tel : " + tel);
    System.out.println("email : " + email);

    System.out.print("Total subject Grade Points = "  ); 
    System.out.print("Total Credit Hours = " );
    System.out.print("Cumulative Grade Point Average =");  //On this 3 system.out.prints, I can't seem to think of a way to read the Grade Point, Total Credit Hours, and CGPA, and add them all together  

}

}