To use a specific ruby version if you have two versions installed you can either specify an absolute path to the one you want. E.g. /your/path/to/ruby Or you can change your PATH setting in your .profile
you can type
which ruby
to see the path to the ruby executable that is used at the moment.
using
echo $PATH
You can see the current PATH setting. You have to prepend the path to your new ruby binary to the PATH so that it will be found before the other one.
As ayaz already mentions, the default location of your macports stuff is in /opt/local. If you add /opt/local/bin in front of your path it should be fine. (Make sure to start a new terminal window after the change - they will not be picked up in your current session unless you explicitely 'source' the .profile file again)
One note of caution: after prepending /opt/local/bin to your path the shell will always prefer binaries in there to binaries found later, this can be an issue if you depend on specific versions in /bin, /sbin or /usr/sbin -- depending on your situation this means that you should not do it (if your computer is processing sensitive data and/or in a bank or something) or just have to remember that it could be an issue (if your computer is a normal development machine).
See http://www.tech-recipes.com/rx/2621/os_x_change_path_environment_variable/ if you need some more hints on how to set your PATH on osx.