Please refer to the contributor guide for all the details:
https://wiki.eclipse.org/EGit/Contributor_Guide
For anything other than small changes, it‘s a good idea to open a bug report for it (in case one doesn’t already exist). This gives others the chance to give input and is useful for tracking. Create one here:
https://bugs.eclipse.org/bugs/enter_bug.cgi?product=JGit
We use Gerrit to review all changes by committers or contributors before they are merged:
Make sure you have an account and have set up the commit-msg hook before committing.
When committing your changes, see the contributor guide or other commits on what your commit message should include.
Run the following to push your change for review (with username replaced by your Gerrit username):
git push ssh://username@git.eclipse.org:29418/jgit/jgit.git HEAD:refs/for/master
Add the link to the review as a comment on the bug report, so that people coming from the bug report can find it.
Then wait for someone to review your change. If there is something to be corrected, amend your commit and push it again.
Have fun :).