From SQL*Plus? You can't trace execution of a single SQL statement (for example, something like "now Oracle selects data from first table, now it performs join, now it is ordering data by some criteria", etc.).
You could, however, trace execution of a PL/SQL block. Not very elegant, but can be done - using DBMS_OUTPUT.PUT_LINE statement where appropriate (for example, before entering the FOR loop and after it finishes, before insert into some table, etc.).
This can be done in a GUI environment using a tool which provides such a functionality; one of them is Oracle SQL Developer, free to download and use.