You need to make entry an entry in the sqlplus global login file.
This file is normally found in $ORACLE_HOME\sqlplus\admin
Its name is : glogin.sql
Add any line to this file that you want to be default for SQL*PLUS
eg to make the unix editor 'vi' the default for SQL add the following line
Next time you enter sqlplus and type 'ed' the 'vi' editor should start.
You can also put 'set' commands in this file - so if you want to change
the default lines per page for add the line set pagesize 25.