I was one of the early adopters for the OSX 10.9 Mavericks update on several of my machines. As I quickly found out, however, MacPorts updating was broken (relax, the existing binaries that were already installed by ports were running just fine).
The primary complaint by the ports installer was the lack of gnutar. If this is your issue; read on. Otherwise, I'm not sure if this is much use to you.
First step is: http://trac.macports.org/wiki/MavericksProblems. There is a link to an installer for Mavericks. This pertains specifically to Mac Ports 2.2.1 - if you are at that version or 2.2.0 then you should be ok to use it. If you're at a newer Mac Ports release, then this post is probably outdated for you. (Hint: issuing "port version" in Terminal should tell you what version you're at).
Run the installer, and you may find that ports will start working again for new updates. In my case, it did not. As I updated to Mavericks, XCode became outdated. So your next step is to download the new version of XCode via the App Store.
After getting XCode up to date, you need to run from Terminal; the command "xcodebuild -license". In many cases, you'll need to append sudo, e.g. "sudo xcodebuild -license".
Finally, you'll need to make sure XCode has the latest command line tools installed. In the new version of XCode, it is not clear where to find it. Fortunately, there's a quicker way: "xcode-select --install" will pull up the dialog to install the latest tools. As with xcodebuild's license step above, you will usually need to run it with "sudo".
Now, run "port selfupdate" followed by "port upgrade outdated".