Firstly you'll need a tool allowing you to translate .po file into your language.

You can use Poedit app, or CodeStyling  WordPress Plugin.
http://www.poedit.net/
http://www.code-styling.de/english/development/wordpress-plugin-codestyling-localization-en

Open .po file from /languages/ folder of the theme, and start translating each word to your language.

After you finished, click button "Save Catalog", and it will create a new .mo file in the same folder.

Then you'll need to open wp-config.php file from the root folder of your WordPress install, and replace this line:

define ('WPLANG', '');

with something like:

define ('WPLANG', 'fr_FR');

Then make sure that .mo file from /languages/ folder of our theme has the same name as your language code: "fr_FR.mo"


More information you can find here: http://www.catswhocode.com/blog/how-to-make-a-translatable-wordpress-theme