You just need to do a domain transfer. I think Dreamhost's signup process allows for that. They will send a request to your current registrar and then you (as the registrant) will receive a notification and request for authorization. Generally you just need to reply to that request via email. The transfer may take a few days to complete.

However, that said, I would not advise transfering your domain to Dreamhost. I just like keeping my domains with a proper registrar and not a host trying to play registrar. Namecheap is a good registrar with a pretty nice control panel and decent prices if you do want to switch to someone else ($8.88 per year)

Bruno