Two ways of finding the tables where are the data that you need.
Probably, the most difficult task for a junior ABAP consultant is to find the tables with the data that he needs to use in a program. Usually, the functional consultant or user doesn't know more than the name of the structure that he can see in the status.
The R/3 ABAP Workbench has two tools that can help us in this issue:
1) The Data Base trace: before you enter in the option that will read the data you want, open a new section and run transaction ST05. Press start trace 'Trace On'. Go back to the other section and proceed with the action. When the data you want appear in the screen, go back to the sectio where the ST05 is on and press 'Trace Off' and 'Trace List'. You will have a list of all tables read by R/3. Probably, it will have a lot of data that doesn't interest you, but with some practice you will recognize fast the data you want.
2) If you know the transaction code or the program name where the data you want is used, you can use the SE49 transaction. It will show all tables read and updated by the program. The only issue here is that the table must be in the TABLES command of that program.
Hope it helps...