Try looking for the SQL that is causing the most physical block reads. They are likely doing full table scans. Then once you have the SQL you then do an explain plan to confirm that full table scans are occuring. If the tables are large, you will want to look at indexing to reduce the scans. Remember to analyze the index after it is built and you will find that your physical block reads will be greatly reduced.
How you do this depends on the tools you have of course. You can do this by looking at V$SQLAREA view, by using the OEM SQL Analyze tool or some other third party tool.
The SQL Analyze tool will also make index recommendations if you want.