You need to do a so-called “301 redirect”, that means when someone (or something
such as a search engine spider) is accessing the page on the old URL, serve them a status code 301
(“Moved”) in order to notify them that the resource has moved somewhere else, and then ideally apply an automatic redirection. This way the search engine robots will reassign their results to the new URL.
This sounds complicated but is relatively simple and best done with an .htaccess file. You create a new file called .htaccess (yes, with period in front) and put that in your web root directory. Here is a tutorial on what to put in that file: http://css-tricks.com/snippets/htaccess/301-redirects/