First of all, Welcome! I think the first thing you need to do is arrange to gain access to the account on Dreamhost. You did not say if another developer was managing the site, or if your client was managing it. Either way, you should arrange to either obtain the appropriate username/password for the Dreamhost account (or at least the ftp user) so that you can ensure the continued integrity of the existing files. Depending on the circumstances of your situation, you may be able to have the existing file/code delivered to you by the owner or developer. Exactly how you should proceed with this depends entirely on “who controls what” regarding the existing site.
Having obtained the files, or in conjunction with doing so via the existing account credentials, I strongly advise that you change the account password(s) in order to maintain the integrity of the account (you should do this periodically, but always after a change where you do not know for sure who has access to the existing password). You accomplish this function, and many others, via the Dreamhost Control Panel, using the existing account credentials. If for any reason you cannot obtain these credentials, you should create a new account on Dreamhost in which to install the existing site’s files. Whether you need to do this, and who will pay for any costs involved, should be ironed out with the site’s owner.
Now having the existing site’s files, and access to a Dreamhost account with which to manage them, all that is needed is to make sure you repeat the process with the domain registrar, as appropriate or indicated by need or security concerns, to ensure that the owner/you retain control of the domain’s DNS settings.
Once you have that all assembled, explore the DH Control Panel, the Dreamhost Wiki, and this Forum as needed to find your way around. SInce you indicated you have experience using ftp, you will probably be okay, though any new environment will take a bit of getting used to, and possibly some additional learning.
If your revious hosting environments were cPanel (or other commonly used commercial “panel”) based, the Dreamhost Control Panel will look foreign to you. Don’t be discouraged by this, and take the time to explore and become familiar with the panel. Once you understand how the DH Control Panel works, you will find that it, in conjunction with the shell access Dreamhost provides for all accounts, provides a web development environment that is far more powerful and flexible than any provided by cPanel.
Your question was a bit “broad”, but if I understood it correctly, some of this may be useful information. If I didn’t understand what you were asking, or if you have more specfic question, just post back (or in a new thread as is appropriate for the subject being discussed) and I’m sure you will users here that will try to help. Good Luck!