Real Ultimate Power is the best ninja site on the internet: http://www.realultimatepower.net/
There is no better place to start your ninja journey.
If you want to become a ninja, there are academies and classes. It is a long journey to reach true ninja-ness, however you will become disciplined and improve your physical, mental, and spiritual fitness along the way.
This wikipedia page includes information about modern ninja schools: http://en.wikipedia.org/wiki/Modern_Schools_of_Ninjutsu
This link contains is a directory of ninja schools by geographic location: