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Doi9t's answer is very good, but even with his improvements, there is still a problem: your code does not produce the correct answer in all cases.

Java strings use UTF-16 encoding. This means that a Java char is not large enough to store all Unicode characters. Instead some characters (for example, 😂) are stored as a pair of chars, and reversing the pair (as your code does) will result in nonsense data. See this documentation for more details.

Fortunately, the way UTF-16 is defined, chars that are surrogates (half-characters) have a completely separate range of values from chars that are Unicode characters by themselves. This means it is possible to test each char individually to see if it is a surrogate, and then have special handling to preserve the pairs.

import java.lang.Character;
import java.lang.StringBuilder;
import java.util.Scanner;
<...>
 private static String reverseString(String myString) 
 {
 StringBuilder reversedString = new StringBuilder();
 for (int j = myString.length() - 1; j >= 0; j--) 
 {
 char c = myString.charAt(j);
 if (Character.isLowSurrogate(c))
 {
 j--;
 reversedString.append(myString.charAt(j));
 }
 reversedString.append(c);
 }
 return reversedString.toString();
 }

If you really wanted to re-invent the wheel, I think Character.isLowSurrogate(c) could be replaced with c >= '\uDC00' && c <= '\uDFFF', though I have not personally tested this.

And while we're on the topic of Unicode, you should of course read The Absolute Minimum Every Software Developer Absolutely, Positively Must Know About Unicode and Character Sets (No Excuses!), if you haven't already.