Create Vigenere Cipher

We have been tasked to create a Vigenere Cipher. At this stage however, I am attempting to get the program to encrypt a phrase using a single letter key (Caesar cipher), before progressing to the final stage, implementing a functioning Vigenere Cipher. The code below is able to encrypt a word, capatalise it, remove white space and shift it a certain amount of places, given the provided letter. My problem is that because the single letter is a String, not a char, the phrase is being shifted incorrectly. For example, when the program is given the Inputs: This is a test with the single letter key A, the program shifts all the characters by 1. The output however should read THISISATEST as A is supposed to be 0.

import java.util.Scanner;
public class VigenereCipher {
public static void main(String[] args)
{
char c;

[code]....