Hi Rableen,
You need to maintain filters in SM19.
EXample can be found here https://help.sap.com/saphelp_nw70ehp2/helpdata/en/6c/57bf393b57ac22e10000000a11402f/content.htm
You specify the activities that you want to log in filters using the transaction SM19. You can read the log using the transaction SM20. You can delete old logs with the transaction SM18.
Hope this helps.
Regards,
Deepak Kori