Oi, talk about stupid.
You want to be a ninja, do the following...
Step 1: Become physically fit. This means you need to run fast, jump high, and have great balance.
Step 2: Learn stealth. Know how to keep quite and out of sight.
Step 3: Know what parts of the bodies you need to strike to paralyze and/or kill your target. This is the part that takes DECADES of training and is next to impossible to learn today, since noone will just teach you where to strike and how. And you won't find out by simply going to Japan, so all you anime fans should guess again. Also, it is best NOT to seek this knowledge.
Ninjas are not master Martial Artists, they are skilled assassins. No Juitsu, no tight revealing clothes. Just those three steps. Breast size has nothing to do with it and can be handled with tape or a sports bra. Also, the force of your kick does not matter. Just the ability to sneek in skillfully and kill without being noticed. Its fun learning steps one and two, but step three should not be seeked by anyone. I never bothered to look. Like I said, very stupid idea. Hell, ask a martial arts place about ninjas and they will most likely tell you the same thing. Oh, and when I said Sneak around is fun, I mean doing it to prank your friends. Don't do anything stupid.