If you get a directory listing when you navigate to the folder that nopCommerce is installed into then it sounds like your hosting isn't configured to properly support ASP.NET
You first need to make sure that this is enabled in your hosting control panel (the latest versions of nopCommerce require .NET 4), you also need an MS SQL Server 2005/2008 database to run the site content from.
Once everything is correctly configured, when you navigate to the installation in a browser it should detect that it needs to install itself properly and will take you through the installation wizard. This is where you need to give information like your database server address, database name, username and password etc.
The installation and full hosting requirements are listed elsewhere on this site :)
Good luck, nopCommerce is great and works a treat