Dear all,
I want to learn something about writing documentation before and after
writing aspx, but i have no idea how to write, any idea how to learn
Your question is very vague, so here's my help...as far as documentation
'before' and 'after', you need to use an approach like RUP or something of
the like, a development (well, also project-based) approach; it can also
be very informal/adhoc. XP is another one, albeit another animal

Approaches have their own documentation, artifacts, etc. but you'll have
to search and decide what's the best for you and bring them
together...usually though documentation involves tech specs,
wireframes/storyboards, class diagrams, etc. depending on how much you
want or need to do.
As far as code/API documentation (after the fact) you can use a tool like
nDoc to auto-generate it:
http://ndoc.sourceforge.net/