Java: Simple Recursion Issue

I'm extremely new to the whole idea of recursion, and it's blowing my mind a bit to be honest. I'm trying to turn this function I wrote into a recursive one...

public static int to_number(String s)
{
    int total = 0;
    int n=s.length();
    for(int i = 0 ; i < n ; i++) { 
        char c = s.charAt(i);
        if (Character.isDigit(c)){
            int value=Character.getNumericValue(c);
            total +=value;
        }
    }
    System.out.println(total);
    return total;
}

It reads in a string, such as "3aaa6a3", iterates through the string, if the char is a digit, it adds it to total, and so on. What I have so far...

    public static int to_number(String s)
{
    int total = 0;
    int n=s.length();
    int i=0;
    if (i == n){
        return 0; //if gone through the whole string
    }
    char c = s.charAt(i);
        if (Character.isDigit(c)){
            int value=Character.getNumericValue(c);
            total +=value;
        }

    System.out.println(total);
    i++;
    to_number(); //trying to call the same function
    return total;
}

I feel like I'm close, but just not getting it. Thanks for your time and effort!