Like @Ardee said you sharer.php uses data from the meta tags, the Dialog API accepts parameters. Facebook have removed the ability to use the message parameter but you can use the quote parameter which can be useful in a lot of cases e.g.
https://www.facebook.com/dialog/share?
app_id=[your-app-id]
&display=popup
&title=This+is+the+title+parameter
&description=This+is+the+description+parameter
"e=This+is+the+quote+parameter
&caption=This+is+the+caption+parameter
&href=https%3A%2F%2Fdevelopers.facebook.com%2Fdocs%2F
&redirect_uri=https%3A%2F%2Fwww.[url-in-your-accepted-list].com
Just have to create an app id:
https://developers.facebook.com/docs/apps/register
Then make sure the redirect url domain is listed in the accepted domains for that app.