That's a huge question.  I'm afraid you'll have to be more specific if
you want a reasonable chance of a good answer.

Wikipedia has a decent article on AVL trees with some pseudo-code and
illustrations.  "Importable files" isn't a commonly used term in Java,
but if I had to guess, I'd think it means you should put this into a
package so that someone else's program can import and use it.  As for
your third question, you could take a whole semester-long college course
on that; but basically, the technique is to count (generally for the
worst possible case) the number of simple operations that are performed
by the function, in terms of the size of the input.  Then express that as
a function, and then perform some simplifications such as dropping non-
leading terms of polynomials, dropping constant coefficients, etc.