You can find out the numerical ID of your Facebook account by just looking at your profile link or by using the Graph API of Facebook.

Some profile links just contain the numerical user ID, e.g.
http://www.facebook.com/profile.php?id=620003169
with 620003169 being the ID in this case

If you have your name in your profile link, e.g.
http://www.facebook.com/martinlormes
you can retrieve the user object in JSON format by replacing the “www” with “graph”, i.e.
http://graph.facebook.com/martinlormes
resulting in output such as this:

{
   "id": "620003169",
   "name": "Martin Lormes",
   "first_name": "Martin",
   "last_name": "Lormes",
   "link": "http://www.facebook.com/martinlormes",
   "username": "martinlormes",
   "gender": "male",
   "locale": "en_US"
}

The JSON data always contains the numerical ID, see my user ID highlighted above.