Hey I’ve just got hosting through Dreamhost and was wondering what the URL is to access the site without a domain name…
I was thinking username.dreamhost.com/etc but it hasn’t worked.

There is no default, because there is no default site on a new account. But you can set up any number of http://$WHATEVER.dreamhosters.com through the panel, which will host a directory or symlink named ~/$WHATEVER.dreamhosters.com after a couple minutes.

If you’re planning on buying a domain name:

Otherwise, yes, create a Fully Hosted Domain called YOURDOMAIN.dreamhosters.com