There are a lot of information on internet and you will find below the steps I have follow.
Into the backend, Click on System -> Manage Stores and configure it like this :
Note the Website Name Code for your apache configuration. here photosart.
Remark: Don’t use an underscore character (‘_’) into the code because It will not work. I lost a lot of times due to this mistake.
You will find below an extract of my nginx configuration :
And It should work as expected 🙂