If I format my imac will I be able to use it?

Will I be able to use it. Or will no files be on it. What I would do is put in my mac cd for leopard. Press C then go to utilities then to disk utility. Press Macintosh then go up to raid. Then put it on Mac osx Journaled. Would I be able to use it? Would there be any files? Would the system defaults be there? I got the update and now I need to go back. This is the only way. Do I need to backup my system?