You need to implement a "301 redirect" on your older domain. You need to confirm if your site is hosted on a Windows or Apache server first. Since you will implement the "301 redirect" the rankings and link juice would be preserved (most of them) to the new domain. Eventually you should use your robots.txt file to disallow bots crawling old domain. When its done, simply go to Google webmaster tools and remove the old domain from Google's index.