By using overrides, there are a number of things that Free Patients can change about their journal appearance. (Early Inmates, "Self-committed" [paid] or Permanently Insane account holders can use the style system, detailed in http://www.insanejournal.com/support/faqbrowse.bml?faqid=43 , as well as using overrides.)
The "HowTo" journal (http://www.insanejournal.com/userinfo.bml?user=howto) have a number of tutorials detailing common override requests. The memories of HowTo (http://www.insanejournal.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.
Please note that detailed customization support is outside the realm of InsaneJournal 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.