Each theme contains a ‘languages’ or ‘lang’ folder where you’ll find a .po file. That file contains the strings that can be translated.

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

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

And here is a video that shows CodeStyling Localization in action

If Poedit is used:
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

If CodeStyling Localization plugin is used:
Just go to Tools -> Locallization , find the theme/plugin you want to translate, edit the .po file and generate a .mo file.
Check the plugin website for more details. Also you should take into account that if the theme is updated, then the translation will get lost. To prevent that, we recommend to use together with CodeStyling Localization also codestyling localization preserver plugin.

Note! If you translate the theme, then keep somewhere copies of the .po files if you are going to update the theme(in case you are not using codestyling localization preserver plugin). This is necessary because when updating the theme all the files are replaced including the translation files. And if you update the theme, upload the backup file in the /languages folder again.