Thanks for the reply. I think baby steps are needed; a lot of that url you posted was completely alien to me!
Here's where I am up to so far (according to emails):
Got hosting with DH
FTP user account 'semsoc' activated
Installed wordpress onto semsoc.com through DH one-click
I can get onto the ftp client 'net2ftp' using the log-in details supplied in an email, putting the FTP server as 'callisto.dreamhost.com'. If I put the FTP server as 'semsoc.com' it doesn't work.
Going into the FTP client with the callisto server, there is a directory called semsoc.com on there. It seems to have all the wordpress stuff in it.
Now, another email said 'wordpress successfully installed on your site'. It has various instructions, the first being 'setup an admin user at http://www.semsoc.com/wp-admin/install.php however, if I click this url, it is a 404 not found page on our old wordpress.com site. Similarly, the other links in the email just go to the wordpress.com dashboard or don't work. I'm wondering, is this because we still have the original wordpress.com blog still up and running? Or is it something to do with switching these DNS? I tried to find how to do that in wordpress.com dashboard but didnt have much joy.
Thanks again for your help