I assume you mean the password in Shinra HQ.
What you gotta do is check all the libraries in that room - there is a panel in front of them that tells you what they are for. Now, search for four books (one for each library) that doesn't fit in with the main subject of the library (e.g. if you see a book about weapons in the space library it's the book you want) check the number at the start of the book title, and find the letter that corispondes to that number - so, if the number is "7" the 7th letter is what you want.
Once you've found them all use the letters you got, and you should get the password, which is different each time.