I believe you'll need to contact the US Patent and Trademark office for that information. As far as the tag line goes, I think that will need to be registered as a trademark as well. This could be very costly - I'm not sure how much.
A good way to protect yourself without "going national" like I stated above is to keep records of your website and logo creation dates for reference purposes. That simple step alone caused a website to be shut down when I contacted the web host and told them that my web copy had been stolen. They asked for dates and that did the trick.
You may not need to do any registering at all, according to this Wiki entry:
http://en.wikipedia.org/wiki/Trademark
Also check out
www.uspto.gov - The Patent and Trademark Office website.
I hope that helps!