I have to create a custom extractor in SAP R/3 system.
What are the ways to extract the data from required tables like COVP, COEP, EKKN, etc.?
I suppose you are asking to extract to SAP BI, although it seems you are probably lacking a bit in experience you'll have to ask an ABAPer to help you with some of this.
First you need to build a view on the DB table you want: go to transaction SE11 and select View, and give it a name. Usually I like to do ZBW_tablename and a Description such as BW Extractor View for Table tablename
Select all the fields from tge database table and activate.
Then go to transaction RSO2 and in this case you are dealing with Transactional tables. Create a transactional extrator by entering a name Z_tablename_TRANS. Click CREATE.
On the next screen, select the position for your extractor in the application component hierarchy. When you replicate into SAP BI you will see your extractor in this position in the hierarchy, so it's important you remember where you put it.
Add the descriptions, and the Viewname that you just created. Select save.
That's it.