By using overrides, there are a number of things that free users can change about their journal appearance. (Paid users can use the style system, detailed in http://www.livejournal.com/support/faqbrowse.bml?faqid=128 , rather than using overrides.)
The journal "HowTo" (http://www.livejournal.com/users/howto) has a number of tutorials detailing common override requests. The memories of HowTo (http://www.livejournal.com/tools/memories.bml?user=howto) are categorized based on which system style they are intended to override. Please make sure that you choose an override intended for the style you are currently using. If you change system styles in the future, you may need to change your overrides.
For important information on overrides, including information on merging overrides, please see http://www.livejournal.com/support/faqbrowse.bml?faqid=124 . For more common customization effects, see the rest of the Customization FAQ category, http://www.livejournal.com/support/faqbrowse.bml?faqcat=customize .
Please note that detailed customization support is outside the realm of LiveJournal Support. If you require further assistance in using HTML or CSS to customize your journal, you can search for "HTML tutorial" in your favorite search engine.