How do you say be prepared in Japanese?

準備 (junbi) is the Japanese Word for preparedness or preparation, so to say "be prepared" as in you are prepared, you say 準備した (junbi shita, "I prepared"). If you are giving someone advice, as in "You better be prepared" it would become necessary, so you would say 準備しなければいけない (junbi shinakereba ikenai) which translates roughly as "If you don't prepare, it's unacceptable", so in this you are saying "you must prepare" or "it's necessary to prepare".