To enable tracing on instance level you have to set the init parameter SQL_TRACE=TRUE and restart the database (I'm not sure, maybe in new releases this parameter can be set dynamically, without bouncing the database).
Once this parameter is set each session will begin to write its own trace file in your USER_DUMP_DESTINATION directory. You can then analyze each of those trace files with TKPROF individually, or you can concatenate them into one large file and run TKPROF on it.
Oracle Tuning manual will give you more details....
Jurij Modic ASCII a stupid question, get a stupid ANSI
24 hours in a day .... 24 beer in a case .... coincidence?
SQL_TRACE=TRUE will start generating the output in Trace file in user dump. TKPROF is a OS utility and will convert the output in a readable format. You can go through the the report (Text file) generated by TKPROF.