Almost every recursion function contains two part : the particular part and then the recursive part.

So to write this function your algorithm will look like this

 if (condition_separate_particular_part) 
    {
        //here the code for part
    }
else 
    {
         //the recursive part 
    }

Now to determine the recursive part you will try to find how to explain the element "i" using it's predecessor elements "i-1" "i-2" ....

Like this it will be easy for you every time.

Note that sometimes it's useful to start to find iterative way to make it easy for your self.

I'm damn sure that you are new so that's why you feel lost a bit. But trust me you will habit soon if you exercise more. Try and you gonna see. ;).

Let me know if you find difficulties to find more exercises/examples. I will try to help you :).

Here is and example. Just type Exercise with solution for recursive functions on C and you will find a lot to exercise ;).

Now here both recursive and iterative code

Recursive

 int fib(int n){
      if (n < 2) // here is particular case
        return n;
      else  // here is the recursion 
        return fib(n-1) + fib(n-2);
    }

printf("%d\n", fib(10));

iterative

int fib(int n) {
  int first = 0, second = 1;

  int tmp;
  while (n--) {
    tmp = first+second;
    first = second;
    second = tmp;
  }
  return first;
}