ok... I want my site to look kind of like this one .. http://www.celestial-star.net/ but I still dont understand how php works.. can anyone explain please? oh.. and also.. how does she make her navigation boxes like that?? Thanks!!

that same page can be created using html and css.
what part of it being php based do you want to re-create?

well, firstly.. I want to learn php... and ive read a few tutorials and I still dont understand what it can be used for!! >_<

php makes a good many different things work...

guestbook,
plugboard,
forums,
form to email scripts,
includes,
counters,
and so forth and so on...

have a look here...
http://www.hotscripts.com
and click on the PHP section. you can see how many different applications may be done using this serverside language.

check out some member's sites from here. they usually have some codes or tutorials or links and stuff that can be helpful in learning this.