hostname & domain name changing

[caa]

hello,
  when i installed my debian box, i only put in some out of nowhere hostname 
and domain names, now it seems that i need to give it some real hostname and 
domain name that really exists.
  can someone direct me please as to what files (all necessary files) that i 
should change for me to go about this.
any help will greatly be appriciated.
caa