First try Caesar Cipher
It would seem that you are restricting yourself to English alphabet, when there is no need for that. All that Caesar cipher does, is adding some constant (usually called key) to each character. Decoding then may be done by subtracting that very key from each encoded character. All in all, I had this in mind:

public class Encryptor {

    public static String encryptString(String input, int key) {
        StringBuilder sb = new StringBuilder(input.length());

        for (char c : input.toCharArray()) {
            sb.append((char)(c + key));
        }

        return sb.toString();
    }

    public static String decryptString(String input, int key) {
        return encryptString(input, -key);
    }
}


Also, using StringBuilder
will improve the performance of your cipher: when you do

char c = '?';
myString += c;


a new String is created which runs in linear time as it needs to copy the character content. However, appending to a StringBuilder of a known size runs in constant time.

I run your program, and it does not cipher the white-space. This makes breaking your cipher easier as the attacker will see clearly the lengths of words. Instead, cipher any character in the input string.