SQL & PL/SQL

Ok, I am working on my first database and I am trying to create an end-user intput to populate my table. I have created a before insert trigger that should automatically populate two fields.
Here it is:
SQL> CREATE OR REPLACE TRIGGER emp_update
2 BEFORE UPDATE OR INSERT ON emptemp
3 FOR EACH ROW
4 BEGIN
5 :NEW.Annual_Salary:=26*80*:NEW.Hourly_Rate;
6 :NEW.Employee_Code:=SUBSTR(:NEW.Last_Name,1,3)||
7 SUBSTR(:NEW.Employee_SSN,-5);
8 END emp_update;
9 /

Trigger created.

SQL>

I'm not very comfortable writing code yet and I need to create a program that will prompt a user to input the other fields in my table (First_name, Last_Name, SSN, Hourly_Rate etc.) Once They input the data I need my trigger to kick off to fill in the two fields that it references.