What martial art should I take to learn how to use sai?

Answer 1

Take a traditional Okinawan or Japanese style of karate. Some have gotten away from teaching weapons or only teach weapons at the advance levels since students have less time to train, study, and work out than students of thirty or forty years ago. Weapons training carries the highest risk of injury for a student, other than fighting in competition, so you will probably have to study empty hand technique first and probably an easier weapon like the Bo before you learn Sai.

Answer 2

No need to take any martial arts. Just watch Ninja Turtles.