Can I use FTP before my domain’s dns propagates?

Yes. Use your IP address found in your welcome email as your “host name” and then your user/password. After your DNS propagates you will want to change the host name to ftp.yoursitesdomainhere.com

What is my username/pass/host name to use FTP?

Your username/pass is the same as in your welcome email. The host name is ftp.yoursitesaddresshere.com