If you wish to add/remove/modify files within your website, you'll need to use an FTP client. There are many inexpensive (or free!) FTP clients available. We recommend SmartFTP or CuteFTP.
Once you have an FTP client installed, connecting is simple:
1.) enter the server name of ftp.yourdomain.com
2.) enter your username and password
3.) once connected, click on the mainwebsite_html link to be redirected to the proper folder
Consult the documentation for your FTP client to learn the specifics for uploading, downloading, making new folder, changing permissions, etc.