There are two really good places to find a port that needs some attention.
You can use the web interface to the Problem Report database to search through and view unresolved PRs. The majority of ports PRs are updates, but with a little searching and skimming over synopses you should be able to find something interesting to work on (the sw-bug class is a good place to start).
The other place is the FreeBSD Ports Monitoring System. In particular look for unmaintained ports with build errors and ports that are marked BROKEN. It is OK to send changes for a maintained port as well, but remember to ask the maintainer in case they are already working on the problem.
Once you have found a bug or problem, collect information, investigate and fix! If there is an existing PR, follow up to that. Otherwise create a new PR. Your changes will be reviewed and, if everything checks out, committed.