To start with PL/SQL or SQL tuning the first step should be to identify the bottleneck in the whole code. Idetifying individual SQL queries that are giving problems is no difficult task.
But identifying some PL/SQL block can be a difficult job.
Oracle has a package called DBMS_PROFILER, with this package , we can
profile a code and idntify certain bottlenecks in the PL/SQL code.
With many Oracle clients such as PL/SQl developer,Toad etc there is an option of PL/SQL profiling.
There is good tutorail on how to use the package DBMS_PROFILER direclty.
These are some good links to follow for that