Find out where the data is stored in SAP is a very challenging task. In some cases, we can find it easily in few minutes, but in another case it takes more then one day to find it.
I will try to give tips to find it out step by step.
1. Technical Information.
Put cursor in screen field, press F1, it will show description about this field. Then press "Technical Information" button, you can get field name and table / structure / view. If you get a table / view field, check the data in table / view, you may find it there. But, if you get structure field, go to next step.
Example of table field: tcode VA03 (display sales order), screen field order (VBAK-VBELN)
Example of stucture field: tcode IE03 (display equipment), screen field equipment (RM63E-EQUNR).
2. Exploring Search help (display available list to help input value by pressing F4 in screen field).
Sometimes field with search help can be found be displaying field in its search help. Example, tcode XD03 (display customer) field Customer (RF02D-KUNNR). It is a stucture field, to find the table, place cursor in screen field, press F4 (display search help), run search help until it show available list. In this list, place cursor in field customer, then press F1 to display technical info. You will get table/view field for this.
3. SQL Trace
Open your transaction , before type anything in screen, open transaction ST05 (SQL trace) in another session, start SQL trace by pressing "Trace on". SQL trace is a tools to record database request. Make sure you don't run other program beside transaction you want to trace. After starting SQL trace then go back to your transaction, run it until you get field to search shown in screen. Then go to SQL trace session, end SQL trace by pressing "Trace off". Then click "List trace", here you will get all table requested while SQL trace on. Explore table listed there, usually stucture field and table field having same field name.
4. Explore based on SAP module.
Go to SE16. Click F4, then click "SAP Application". Here you got table tree groupped in SAP module.
5. Find it in source code.
Well... you already finished up to step 4, but didn't find the table source yet. It's time to take a harder step. Click F1 then technical information to get program name and field name. Then go to SE38, open the program, then find field name. May be your detective instinct can help out there.
6. Debugging abap program.
Debugging abap could be a sweating task. "Watchpoints" will help you very much in debugging. Considering it could be more than one day to accomplish this task, don't forget to capture and create documentation while debugging when you reached an interesting point. I always do that, when I saw good point, I capture it by "print screen", paste it into Microsoft word, then give a remark on it.
Here is another tips that could help you.
1. Adjacent table will have similar prefix:
sample in sales order, prefik ( V / VB ) : VBAK, VBAP, VBPA...
sample in material management ( M ) : MKPF, MSEG, MARD, MARC,..
2. Almost all SAP Table have a view
To find related table, goto SE11, click where-used list, choose Views.
3. Another tools to find table field:
Goto SE15, Choose: ABAP Dictionary -> Field -> Table Fields.