When you add the domain on "manage domains" in the panel, one of the things you specify is user. If you already have added the domain then the user will be displayed in the web hosting column on the manage domains page. The password is determined when the user is added, if you don't know the password for the user in question, you can use "manage users" to change it. Be aware that if you change the password it will take 15-20 minutes for your changed password to become effective on the server.
Yes, port 21 is correct.
A second thing that happens when the USER is created or changed, is that a directory with the same domain name you are adding is added to that server users files. When you successfully log in to that user via ftp you will see that directory, named yourdomain.com -- switch into that directory. This is your live file space for the domain. If you uploaded and index file here, it would be used for the domain.
There are a number of free ftp client, my personal favorite is winSCP there are others listed here.
yes use it to transfer the site into the domainname.com directory that was created for you. If you upload it to any other directory it won't work.
3 - there is no step here. The domainname.com directory that is added for you IS live, and changes to it are reflected immediately.... well sort of... don't forget things like the browser cache and anything like cloudflare that might cache things. (hint: if you turned on cloudflare, turn it off for now.... get your site working first...then add that later if you really need it.) Ctrl-f5 (server reload, no cache) is your best friend when you are working on a site.