First you'll need an FTP client. A nice google search will turn up hundreds of programs to use. Once you have downloaded an FTP program, we suggest that you read the documentation in order to become familiar with its. Some examples are:
You'll need to enter the 'hostname' of your site (often called a domain name or server, depending on the FTP program). For example, if your site is viewable at 'www.yourdomain.com', the hostname would simply be 'yourdomain.com'.
If your domain hasn't been set up yet (not registered or not transferred to us) you can still upload your site directly to your server hostname or IP address. You should have received those in a welcome email.
Next, you'll probably be prompted for a username and password. These values should have been given to you when you first signed up. If you have any questions as to what they are, please open a support ticket.
When you login via ftp, your web root is public_html folder. This is the folder where you need to upload your web site files.