Your system probably uses ASCII. In ASCII, the codepoints of lowercase characters and uppercase characters are sequential.

So we can just do:

void to_upper(char *message) {
    while (*message) {
        if (*message >= 'a' && *message <= 'z')
            *message = *message - 'a' + 'A';
        message++;
    }   
}   

Other character encodings can become much more complicated. For example, EBCDIC doesn't have contiguous characters. And UTF-8 introduces a world of problems because of the numerous languages that you need to support.

There is nothing like string in C

The best you can do declare the string is like this:

char greeting[] = "Hello";

or like this:

char greeting[6] = {'H', 'e', 'l', 'l', 'o', '\0'};

Check out this tutorial.

The string in C programming language is actually a one-dimensional array of characters which is terminated by a null character '\0'. Thus a null-terminated string contains the characters that comprise the string followed by a null.