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.
Sorry to hear about the problem.