You can probably start by saving and deleting journal receivers. Deleting old spooled files – you can also try a RCLSPLSTG to reclaim spool storage. Save and clear the QRPLOBJ library. Save and delete QSYS/QHST* files.
This should help you get started.
1. database files in developer libraries and size of developer libraries
2. mutli-member databases with obsolete members
3. large database files with a lot of deleted records – rgzpfm, this requires exclusive lock on database.