> (1) Finding a substring (say, aString ='efg') within the main string
> (eg, mainString = 'abcdefghijklm'). The mainString will have NOT have
> a fixed range of characters.
>
> I've tried using the index of, but I think I'm doing it wrong. What I
> want is to be able to see if the subString matches ANY portion of the
> mainString.

int i = mainString.indexOf(aString);
if (i != -1) {
// Found an instance of aString in mainString at 'i'.
}

If you need to find multiple instances of aString in mainString then
you can do the same thing but in a loop until you reach the end of the
string.

>
> (2) I know how to get length of a string, but is there a way to
> answer in an integer format (which I'd use in a FOR statement later
> on)? Eg. I get a length of 5 - how can I convert that into a integer
> variable. Note: I don't want to alter the String! Just get a integer
> output for length.

int length = str.length();

length will contain an integer value the represents the length of the
string 'str'.

BTW, To convert a string representation of an integer:

String str = "14";
int i = Integer.parseInt(str);

'i' will now contain the value 14.

>
> (3) Convert a String into separate characters. Eg a string
> of 'abcde'. would be converted into five DISTINCT characters A B C D
> E that I could then perform calculations against. Again it's not
> fixed.
>

char c[] = new char[str.length];
str.getChar(0, str.length, c, 0);

> Any help  of this would be most welcome.

I would suggest that you get the JDK API docs.