You can list out the files in each of your libraries by using the wrkf (work with files) command. Key the command on the command line and press F4 to prompt it. Then press F1 (Help) and F2 (Expended help). This shows the documentation for the command and how to use it. You can do this for any command. You can also just press F4 on a blank command line to access the online help for all commands (very useful). Since you said you were a “newbie” i suggest you learn the AS/400’s command language. It’s very powerful and intuitive once you get the hang of the grammar. You can also write programs with it.
Here’s some examples for your question…
If you know the library name you can type:
wrkf <library>/*all pf
This will display all physical files (these contain your data) in the library.
Or you might want to see all the user files on the system. In that case you could run it this way:
wrkf *allusr/*all pf
Once you know where your data is, you might want to start looking at your logical files which are can be composed of keys, filters, field subsets, joins, etc. and are based on one or more physical files. They don’t contain any data. You’d use the same command as above substituting “lf” for “pf”.
There are plenty of other tools you can use, but hopefully this will get you going. You may also want to look for files with these names: QTXTSRC (text), QCLSRC (CL source), QDDSSRC (Data Description Specs source), QRPGLESRC (ILE RPG source) and QRPGSRC (OPM RPG source)… assuming there is source to the programs and files. It sounds like you don’t have any documentation. You should see if you can get some because it can be mighty hard to piece everything together without it… but it can be done.
Hope this helps!