"prepared" as an adjective = "ready"
The meanings of a) "I need to prepare" and b) "I need to be prepared (adjective)" are a little different.
In a), I am not ready. I need to do something in order to be ready. [This is about an action]
In b), I am not ready. However, I need to be ready. [This is about me and my state of readiness]
"Prepared" is a common adjective and "to be prepared" is a common collocation.
In the sentence below, "prepared" is part of a passive verb construction:
"I need to be prepared by my teacher for the exam."
In other words, "I need my teacher to prepare me for the exam" (active form).
This usage of "prepared" is less common than the adjectival use.
Here are some other examples of the use of "prepared" as an adjective.
1) The motto of the Boy Scouts of America is "Be Prepared." It means always to be ready for anything.
2) "Mustard" can mean a dry yellow powder made by grinding the seeds of the mustard plant. It can also mean a yellow sauce or condiment made by mixing dry mustard with vinegar, water, salt, and other spices. In everyday speech we call both of them "mustard," but more formally the mixed sauce is called "prepared mustard."
3) A famous quotation from the bacteriologist Louis Pasteur, is well known in English translation as "Chance favors the prepared mind." "The prepared mind" means a mind that is ready to notice and pay attention to unusual things.
Ah, now I see it as adjective on dictionaries. That means I wasn't careful enough when I looked it up.
Thank you, Michael Chambers. :)