Just return i for index of character in string as :

public static int findInStr(String s1, char c){
for (int i = 0; i < s1.length(); i++) {
    if (s1.charAt(i) == c) 
        return i;
}
return -1;

}

You can also use this method without using for loop::

public static int findInStr(String s1, char c){
    return s1.indexOf(c);

}

If character is not present in string indexOf return -1 otherwise index of first occurrence of character

After you've found the first index, use the overloaded version of indexOf that receives the start index as a second parameter:

public int indexOf(int ch, int fromIndex) Returns the index within this string of the first occurrence of the specified character, starting the search at the specified index.

Keep doing that until indexOf returns -1, indicating that there are no more matches to be found.

You can get away without the index variable as you're reassigning it at each step of your loop anyway.

public static int subStringIndex(String str, String substr) {
    int substrlen = substr.length();
    int strlen = str.length();
    int j = 0;

    if (substrlen >= 1) {
        for (int i = 0; i < strlen; i++) {              // iterate through main string
            if (str.charAt(i) == substr.charAt(j)) {    // check substring
                j++;                                    // iterate
                if (j == substrlen) {                   // when to stop
                    return i - substrlen; //found substring. As i is currently at the end of our substr so sub substrlen
                }
            }
            else {
                j = 0;
            }
        }
    }
    return -1;
}