The previous post is very appropriate.
If you're using a hardware RAID controller; generally you'll be able to set this up when the machine boots (usually you'll have an option to hit one of the function keys to go into the controllers configuration) - but this is dependent on your controller itself.
If you want to software RAID, this has everything to do with your partitioning. You could choose to configure RAID 1 against specific partitions or against the entire drive (if you have 2 drives say).
Either way, you need to be more specific for a more concise answer - give details on exactly what equipment you have and what you want to achieve.