How Do I Add a Backup Domain Controller?

Ok here is the situation. We currently only have 1 DC, and I want to have some sort of backup DC for redundancy in case the primary DC goes down. Is this a secondary DC I need to add, or is it a replica DC? Or do those two terms mean the exact same thing?

I basically want a backup DC in case the primary goes down. I know I have to use dcpromo, but I am rather new at this and need to know what steps I need to take…

If someone could offer step by step instructions, I would greatly appreciate it…

Jho