First you need to download an FTP client software. If you don't have one, we recommend Filezilla

When you start your FTP client, you need to specify the FTP hostname to connect to. Your FTP hostname can be found under your 'Account' page. You also need to specify the FTP login details which are the same as your user ID and password.

Here's a good beginners guide on how to use Filezilla: http://www.ostraining.com/blog/coding/filezilla-beginner/