The best teacher is experience. Find a job in the field that is proactive and go for it.
Most of the time, you will find, the best option is no option...if you have to use force, or are in a position where you are defending yourself then chances are something went wrong. Simple arrests aside, your best friend is the interaction between your brain and mouth.
There is no real martial art geared for law enforcement. In fact many law enforcement systems take snips from many different martial arts, but the fact of the matter is you have to work hard at deciding what works and is comfortable for you.
Find a partner to work with and take it from there.