Reading / Sorting And Writing A File

I need to read lastname, firstname, and ssn from a text file and sort them based on oneof those three creiteria. Here is my person class:

public class Person implements Comparable {
private String lastName, firstName, ssn;
private static int sortBy = 1;
public static final int LASTNAME=1, FIRSTNAME=2, SSN=3;
public void setSortMethod(int method) {
if(method != LASTNAME && method != FIRSTNAME && method != SSN) {
throw new IllegalArgumentException();
} else {

[code]....

how to sort my info and store it into an array and then write that info into my new file.