If you want to start contributing changes, then the first step is to checkout the source
code and build Apache Tomcat. See the Building page in the documentation area for the
version of Apache Tomcat you want to work with for detailed instructions.
When making a contribution, minimize unnecessary or unrelated changes (such as formatting).
Apache Tomcat has many contributors, and being able to diff changes throughout the source history is important.
Making small gradual changes and respecting the conventions of the existing source code
will make it much more likely that your patch will be accepted.
Once you've got Apache Tomcat up and running, then the Apache
Contributors Tech Guide is a good place
to learn how to create and submit patches.