After creating your network, the site title will be named by default 'Your Site Name'.
The 'General' page is where you can edit your site title. You will find this under 'Setup' in your Admin Panel when you log in.
In 'General' not only can you change your site name, but you can also add a subtitle to your site. There are also options to change your sites primary language and to change the timezone.
When finished, click 'Save Changes' for your website to be updated with your new name.