I highly recommend figuring out RVM because switching back and forth between different versions of Ruby (and Rails) is a breeze.
If you installed Macports correctly, RVM should be easy to install with the following commands:
$ sudo gem install rvm
$ rvm-install
If you are having issues with MacPorts, check and see if your PATH is set up properly. I had to add this line to the top of my .bash_profile file (which is a hidden file in my home directory):
export PATH=/opt/local/bin:/opt/local/sbin:$PATH
You can read more here: http://guide.macports.org/#installing.shell
Once MacPorts and RVM are installed, you can add versions of Ruby like so:
$rvm install 1.9.2-head
$rvm install 1.8.7
I didn't install 1.9.1 but it's probably '$rvm install 1.9.1'. Remember not to use 'sudo' to install ruby after you've installed RVM.