If you have a Facebook page like this:
https://www.facebook.com/something
And you have a need for the ID of that page, you can type the following in your browser:
http://graph.facebook.com/something
That will return something like this:
{
"id": "123456789012345",
"about": "whatever",
"can_post": false,
etc,
}
That “id” value is what you seek. If you pass through the name of a page that is really an alias shared by many pages, the result will return display an error. Instead, you can look at this other article to try other methods of finding the ID.