An FTP client is used to transfer files around in your website.  You can get that for free here:

http://www.smartftp.com/ or

http://filezilla-project.org/

Don’t worry if it looks complicated.  Hostgator has 24 – 7 customer support online and they can walk you through anything you may need regarding your ftp client.

After you have purchased your domain name and have a hosting service (hopefully Hostgator).  You will unfortunately have to wait about 3 to 4 days for your domain to propogate.  In the mean time we will be researching your niche and finding keywords.