Use for loops and have a look at java.lang.String (the api documentation on
for instance http://java.sun.com ).

import java.io.*;

public class TriangleString{
public static void main(String[] args){
try{
InputStreamReader isr = new InputStreamReader(System.in);
BufferedReader br = new BufferedReader(isr);
String s = br.readLine();
while (s.length() > 0){
System.out.println(s);
s = s.substring(0, s.length()-1);
}
}catch(IOException ioe){
System.err.println("IOException: " + ioe.getMessage());
}
}

}