What are the different types of conditional processing statements that
are available in PL/SQL? Describe when each might be most useful.

This sounds like a homework question.  You can read all about pl/sql
in the manual.  Go to otn.oracle.com and go to documentation.  You can
look up the list of books and I would choose the pl/sql reference
quide.  It has all about the language etc.(you might have ot register,
but registering is free and they do not spam you or give your name out
to others.)