How do I get a google voice number?

A while back, and once again more recently I heard about google voice where you could call people,etc. and tried to sign up. I went the site and pops up with a box that says choose your number, i put in the zip code and there are none for my city. Does it matter what city I choose? Do I have to have an invite to use google voice?|||It doesn't matter where your city is located. If you have ever used Skype for a call. They base their calls out of Washington d.c. You would obviously just have to deal with people questioning if they know someone from whatever area you choose.