Let's face it. Everybody wants to be a ninja. But not all of us are lucky enough to be born in fourteenth century feudal Japan and/or as an anthropomorphic turtle named after an Italian Renaissance painter. So how would one become a ninja in modern times? It's all about the attitude. Sabotage, espionage, and assassination are optional--and in many cases, not encouraged.
First and foremost, ninjas are known for their stealth and invisibility. Part of that is in developing one's own sharp reflexes, grace, night vision, and ability to move quickly in silence. Think about whether you want to be an indoor ninja or an outdoor ninja, and start practicing right away.
Next is the outfit. Conceptually, part is tough. You can easily go on the internet and buy a ninja outfit online, but then you have to come to terms with the fact that you are a commercialized ninja. Just as James Bond is clothed by Brioni, you will be clothed by karatedepot.com. It might undermine your ninja street cred.
Another option is making your own outfit. You could either kidnap someone and have them make it for you--using either threats and intimidation or seduction. Or you can learn to sew and make it yourself. Spider-Man did it. No shame in going to a fabric store.
Outfits need to be light and maneuverable, as you will be doing many incredibly awesome moves in them. Generally, ninja outfits are depicted as being all black, but historically, there is no proof that this is true. Generally, they are in a solid color that is useful for camouflage. Ninja boots need to be sturdy, comfortable, and virtually silent. Head covering must be secure and cover the entire face except the eyes.
Ninjas are warriors. Master a few weapons and be a master at trickery. Attach a sword onto your back. Become swift and accurate with ninja stars. Nunchucks, sais, bo staffs, swords and vanishing bombs are highly recommended.
Get a mentor.
Ninjas are often depicted as spies to learn secrets from their enemies, and therefore gain the upperhand in combat. Think about who your enemies are, and where you could disguise yourself in order to obtain undercover information.
Be prepared for each of your missions. Often times, obstacles will arise which will throw your plans off course. If you're prepared, you will be better equipped to handle these spantaneous diversions.