Q:

How to use utlizie stringbuilder to add every other character and restart at a space? 

The half string should return every other character in a string it should return the starting character.  For instance, halfOfString(“Qbkm Zgis”, 0) returns the String “Qk gs” and the call halfOfString(“Qbkm Zgis”, 1) returns the String “bmZi”.  Once you get to a space the letter coming after should be counted then you continue taking every other letter.  
My code is not working the way it is supposed to and I've been trying to fix with different ways but a space character throws everything off. 
 public String halfOfString(String message,int start ){
    StringBuilder halfString = new StringBuilder();
    int length= message.length();
    for (int k=start;k<length;k++){
        char achar = message.charAt(k+1);

        if (Character.isLetter(achar)!=true){
            achar='\0';

        }

        System.out.println("this is char"+"\t"+achar);
        halfString.append(achar );
        System.out.print("this is halfString in the forloop"+"\t"+halfString);

    }

    return halfString.toString();

}

A:

You need to do chatAt(k) instead of k+1, and you don't need to check to see if it's a space.  Also do k += 2 instead of k++ (As you want every other character).  Lastly you don't need to resolve the length to a variable.  You can just check the length directly:
 public static String halfOfString(String message,int start ){
            StringBuilder halfString = new StringBuilder();
            for (int k=start;k<message.length();k+=2){
                char achar = message.charAt(k);
                halfString.append(achar);
            }
            return halfString.toString();
        }

Output:
//with 0 as start:
Qk gs
//With 1 as start:
bmZi

