You can use the suggested template from RUP (Rational Unified Process); if
you buy it, you will get the template with the suggested structure to create
the software architecture  description document.

For now, I will suggest  you an initial software  architecture document
structure (see below), so you can start working on your assigment. In fact,
software architecture is not mature enough to have a cook book for different
aspects, but learn about the 4+1 View Model from RUP and The Cube Framework
from SunTone Architecture Methodoloy. I hope this will help.

I have a IT consulting company on Software Architecture, Lucasian Labs and
we provide services  for software architecture description and assestment; so
if you need some help, please let us know. We still do not have our website
in english (just spanish), but you can check our consulting services at:

http://www.lucasian.com