To begin with, there are several different versions of ninja warrior that you can compete on, most notably American Ninja Warrior. For those in America, it makes the most sense to try to get on American Ninja Warrior because it is the most local competition. However, American Ninja Warrior is only open to American residents over the age of 21 (this has been an issue for several competitors like myself who are not yet eleigible to compete because of our age). Therefore, if you do not fit the criteria, you may also want to apply to Ninja Warrior (Sasuke) in Japan, because there is no age limit, and the competition is open to anyone from around the world. On the other hand, there are is also a version for Europe which is still being developed, as well as spin-offs in Singapore and Malaysia which often are tied to the Japanese version in regards to qualifying. That being said, below are the details for how to get on each version of Ninja Warrior.
The way to get on American Ninja Warrior is by making a submission video and submitting it to here. The criteria varies each year and can also be found at the link. Most recently, competitors had to make a video 5-8 minutes in length. As mentioned before, you must be 21 years or older and a legal resident of the U.S. You will also need to complete an application found at the submission site including a digital photo of yourself.
Some important tips I would give would be to
1) Make sure you sell yourself to the camera. As a TV Show, the producers are very intent on showcasing personalities as well as good competitors. Having an inspirational story, showcasing your family, owning a unique collection of items, or any other interesting component about you and your life will go a long way in helping you get picked. Very often, interesting personalities will win over good competitors.
2) Be exciting (visual and vocal). Film every aspect of you and your training. This includes your personal life, family, any obstacles you built, and your hometown. Demonstrating hometown proud has been critical to making a good submission video.
3) Create a quality video. This means good lighting, steady filmng, quality audio, and perhaps some interesting transitions and editing to make your video stand out
One of the lesser known competitions, Sasuke Malaysia applicants can fill out a form and apply here. The criteria includes being 18 years old or older and a citizen or permanent resident of Singapore.
Another smaller spin-off, sasuke malaysia is a two stage course. You are required to be at least 16 years old and a legal malaysian resident. Details of the competition and how to apply can be found at the application section of their website.