Problems changing theme (layout)

hello,
i am a newbie from germany. I have the following problems. i use the “x blog” theme and want to change some things. Some things i could already change although – to be frank – sometimes i didn`t know what i was doing as i am not very experienced in css and php.
my page: http://www.birgit-giering.de/wordpress
i would like to change the navigation: instead og “home” “Start”. Impressum at the very end of the navigation. Archives / categories not in blue but in white or black or not there at all.
i already looked in all the theme files, but i couldn´t find anything that looked promising. what do i have to do where?
thanx
snoopy64