I would highly recommend taking the time to learn LyX [http://www.lyx.org] rather than LaTeX itself unless your dissertation will contain a lot of equations. LyX is now a pretty mature piece of software that makes working with LaTeX very easy while still giving you access to all of its advantages.

In any regard, for the love of god, please don't use Word or OO.org, you WILL regret it :-)

The Not So Short Introduction to LATEX2ε

http://www.google.co.nz/search?q=lshort&ie=utf-8&oe=...

For the general LaTeX user, the WikiBook on LaTeX is pretty useful.

http://en.wikibooks.org/wiki/LaTeX

At the beginning I learned most of it from the Not-so-short introduction (link in another reply to your comment), but LaTeX is too large an animal to handle it with a reference manual. I find most of the answers I need on the web.

Also, you probably will have to present your research at seminars/conferences etc.; I'd suggest you take a look at the Beamer document class for LaTeX.