Well, suppose you have the string "GIRL". A string is an array of characters. Store the letter you want to search for as a character. So what you will have to do is start at the beginning of the array, and recursively traverse it one character at a time. We look at the first character. Is it equal to our search character? If yes, increment our counter. Move to the second element (character) in the array. Check if it is a match. Then move to the next character, etc.
if c='a' && 'A'
.....to if c=z...
and one more question
when more than one characters have the same frequencies
how can i take out the earlier one ?
That won't neccesarily work. You're going to need to make a loop and a lot more code to actually search a string. If you are talking about big strings you might want to search for a substring with strstr(). Then get the position of the character you are searching for by finding the difference in the subtring and the string itself. After that, you can use a specific tallying method to loop through that again but skip over the first appearance and go to the third, then fourth, etc... Until the end.