Re-read Shiro's post and then come back to this one if you're still stuck (he gave you the answer). Back already? Okay. Have a 'total' variable, initialized to zero. Increment the value of 'total' by the value of the first character in the string:
Code:
total += my_string[0];
Now do the same thing for the rest of the characters in the string. This is an ideal place for a for loop.