Hi, Welcome aboard Andrei!
If you follow the "bugs in pkg-java public packages" link from https://java.debian.net/ you will find a nice stash of FTBFS bugs (those with the ⛺ symbol). Working on a few of them is probably a good way to become familiar with the packaging and build systems as well as Debian's peculiar BTS.
Some of these FTBFS bugs were probably resolved in a way or another but were never updated or closed. Outdated FTBFS reports will unnecessarily prevent packages from being shipped in the upcoming stable release, so clearing them is useful.
I would suggest you to first try to reproduce the issue reported in the bug with the current version in `unstable`. Pay attention to the package version and for now skip bugs reported against experimental packages, bugs that are already fixed in unstable, and FTBFS on specific architectures.
Try to aim for bugs that are between 3 months - 3 years old as those are the most likely to be outdated and/or easily resolved. Also look for bugs that have FTBFS in the title but are not tagged "ftbfs", they need tagging if they are reproducible.
Setup (for example) sbuild in unshare mode with an unstable chroot, try to build a good package with it to check that it works, and then use it to try to build packages with open FTBFS reports to check if the reports are still relevant.
Then take notes and report here or on #debian-java on IRC. Most of these bugs will need to be reassigned to a different (dependency) package that is actually the root cause of the issue, try to figure out which one and if the build is now successful which version of the package or the dependency solved the issue (by checking the changelogs or other similar resolved bug reports).
Good hunting! Cheers, -- Julien Plissonneau Duquène
