The first thing to do is to download a product called akeeba and make a backup of your site and copy it to your local computer.
Then if you do not know how to set up a multi language Joomla site I suggest that you look at this video.
Once you have done that and set up the base of a multilanguage site it is fairly easy to copy all the menüs and the articles ‘en mass’ to the 2nd language. Of course you then have to translate everything – joomfish does not work yet on J2.5.
Hope that helps you start and really seriously do that backup first – if something goes wrong on the way you do NOT want to lose all the work you have already done.
Good Luck!
