If you are allowing your user to input directly through SQL Plus, I would suggest giving them a script to run rather than having them execute a package/procedure. Create a "test.sql" file as follows:
Code:
accept a prompt "Input First Name?"
execute my_procedure( '&&a' );
You can add whatever else you need; after that, from SQL Plus they would type:
SQL> @test.sql
However, HIGHLY suggest having users go through a program interface rather than directly through SQL Plus...
JoeB