Install the Spanish language files for the front end.
A) Set Spanish as the default language If users need to register on your site, when the register, they'll be able to choose the front end language that want.
Installing additional language files, in your case, Spanish will provide the translation for 'anything Joomla', any components, modules, or plug-ins, menus, navigation links, etc. that are native to Joomla.
It won't translate content that you create; menus names, customs modules, articles, etc.
If you want to have a multilingual site, you'll need a content manager for such as Joom!fish.