Check that you downloaded the most recent version of Steam client.
Check your email for a validation key/link.
I believe you can create an account on their website? If you are, it could be a website bug or server issue, which'll likely be fixed. If not, it could equally be a client bug, which again, they should fix within the next 24 hours - if you haven't tried doing it via the website give it a go. Steam isn't always the most reliable application (though it's still great in most cases) so it can be a bit buggy.
I had the same problem downloading Portal 2 (Stuck on 'Creating Account'). This was after entering my new username and password along with my email address. Here is how I finally managed to fix the problem:
Close Steam. Ctrl-alt-delete to run task manager and close the frozen Steam application
Check your email and click the link to verify your email address
Go to the Steam website and log on. The first time I logged on it said there was a problem verifying my email account. I logged off and re-logged back in and problem seemed to resolve itself.
Try to run the game again. I was prompted by the Steam program to login. Everything worked fine and I was able to download the game and updates.