Taking over maintainership of ports that are unmaintained is a great way to get involved. Unmaintained ports are only updated and fixed when somebody volunteers to work on them. There are a large number of unmaintained ports. It is a good idea to start with adopting a port that you use regularly.
Unmaintained ports have their MAINTAINER set to ports@FreeBSD.org. A list of unmaintained ports and their current errors and problem reports can be seen at the FreeBSD Ports Monitoring System.
Some ports affect a large number of others due to dependencies and slave port relationships. Generally, we want people to have some experience before they maintain such ports.
You can find out whether or not a port has dependencies or slave ports by looking at a master index of ports called INDEX. (The name of the file varies by release of FreeBSD; for instance, INDEX-8.) Some ports have conditional dependencies that are not included in a default INDEX build. We expect you to be able to recognize such ports by looking through other ports' Makefiles.
First make sure you understand your responsibilities as a maintainer. Also read the Porter's Handbook. Please do not commit yourself to more than you feel you can comfortably handle.
You may request maintainership of any unmaintained port as soon as you wish. Simply set MAINTAINER to your own email address and send a PR (Problem Report) with the change. If the port has build errors or needs updating, you may wish to include any other changes in the same PR. This will help because many committers are less willing to assign maintainership to someone who does not have a known track record with FreeBSD. Submitting PRs that fix build errors or update ports are the best ways to establish one.
File your PR with category ports and class change-request. A committer will examine your PR, commit the changes, and finally close the PR. Sometimes this process can take a little while (committers are volunteers, too :).