Use GetCurrentFolder method to get the root folder name as soon as you are logged in:
using(Ftp ftp = new Ftp())
{
ftp.Connect("ftp.server.com"); // or ConnectSSL for SSL
ftp.Login("user", "password");
string root = ftp.GetCurrentFolder();
ftp.ChangeFolder("subfolder");
ftp.ChangeFolder(root);
ftp.Close();
}