Assuming you are using only simple standard system tools (no additional nor third party software) there are three main points you can start in greenscreen:
- go backup
- go save
- dsplog and wrksplf
In the "backup" menu (1st option) you can check the status of backup (DSPBCKSTS) and redefine or run one (RUNBCKUP).
In the "save" menu you can make backup copy (commands begining with SAV*) of specified objects from the system. Restore is done using one of the RST* command.
Using dsplog (system log) you can check any error messages logged by application; and using wrksplf you can usually reach very detailed reports of almost any task.
If you are not very familiar with AS400 environments, the best (and probably the fastest) option will be to start with reading few chapters from IBM Information Center http://publib.boulder.ibm.com/iseries/