Many talked about the physical part. Meaning the ability to technically be "proficient" enough to shoot well in a dynamic shooting scenario.
There are at least 2 other dimensions.
Mentally one has to be ready for the ramifications of lethal force... the use of it and the after effects. How you as a person will deal with it.
Third is the financial side. Many people truly don't know nor prepare for this at all. Here's a group you might be interested in...
http://www.armedcitizensnetwork.org/