help on creating shell script

Can some one smart here help I need to create a shell script that does this below using vi editor.

decrypts a file (specified as an argument to the script) containing text which was encrypted using the ROT-13 algorithm. The decrypted text should be written to a new file leaving the input file unchanged. An error message should be displayed and the script terminate if the input file does not exist.

Doable with tr(1)... Bah, tell your teacher to google it.