how can i come in to the secret cave that rachel told me and whats the comenationtothe safe???:S

You have to get to the place called "archive". then find the way up somewhere on the ladder. Look up and you'll have to perform bird flip, then just follow the way.

After you reach the monk's place, you'll find a book that contains date and directions. They were marked by a green font color. Just follow the direction and make a combination of those numbers on the date. The safe contains a book, then you must place the book in front of the altar.