With 100 sites, you should have at least a clue where to begin...
Does DH host your domains? Do you want to move them too? Read about transferring domains. Do you want to let DH continue as your registrar? Read about pointing your domains to your new host.
Do you run CMS sites with files and databases? You need to back-up and move anything that holds your content/customizations. Use the shell or ftp client to get to your files and use the panel to get to your databases (credentials are all found there, too).
Do you have new hosting? It's easier to move once your new hosting is set up, just read up about using the shell to move content from one server to another.