Your code is working fine.Let me explain what is happening in your code.
first you have initialized the employee name = null , his salary and age =0 and made this to print in a .txt file.
So the ".txt" file contains
null
0
0
and you have asked the user to enter the data which is not useful.
so you have to remove the initialization.(for your reference code given below)
String employeeName;
String jobTitle;
double salary ;
int age;
and cut the data which ask the user to input his details and paste it above here
System.out.println("Please enter name: " );
employeeName = input.next();
System.out.println("Please enter salary:");
salary= input.nextDouble();
System.out.println("Please enter age: " );
age = input.nextInt();
System.out.println("Please enter job title: ");
jobTitle = input.next();
input.close();
try{
FileWriter fw = new FileWriter("employeFile.txt");
PrintWriter pw = new PrintWriter(fw);
pw.println(employeeName);
pw.print(age);
pw.print(salary);
pw.print(jobTitle);
pw.flush();
pw.close();
}