Hopefully your school has some premade templates and styles for their dissertation format. Those might be in Word, LaTeX, or something else entirely. If not, you'll have to go through their style guide and work it up yourself. I'm not a Word user, but the advice I've heard from those who are would be to use styles heavily for structural elements and other formatting.

Example: a chapter title should have a "Chapter Title" style, which is defined according to the rules of your school (maybe that's bold, all caps, centered, 14pt). If you make a mistake, you update the style definition once instead of updating every title's formatting manually.

Your school probably won't care what tool you use to compose your dissertation, only that it matches their format and style. Your advisor might be more strict. Your library may want an electronic copy for archival purposes, but they'd probably want a PDF. I did my master's thesis in WordPerfect. Others used Word, LaTeX, etc. I'd expect you could use Pages, OpenOffice, or anything else that has the required level of support for graphs, tables, or equations as needed.