There are a number of ways one can contribute to the GeoTools project.
The project mailing lists are the best and way to get help with GeoTools as well as stay in touch with what is happening with the project.
The developer list is meant for discussions that pertaining to the actual development of the geotools library. If you have a patch you would like to contribute, or an issue with a plug-in you are working on, discuss it on this list. This is list is also open to anyone who would like to take part in the more day to day development discussion.
The administration list is for the care and feeding of the GeoTools project.
Reporting bugs is one of the most valuable thing a user can do to contribute to an open source project such as GeoTools. GeoTools uses a system known as JIRA to manage bugs, issues, and new features. If you have found a bug, or if there is a feature that you would like to see implemented in GeoToools, please report it in JIRA.