Auto Populate Contact Informaiton when User Account Number is input

Good Morning,

I am just getting into PL/SQL and I wanted to set up my Application to automatically populate the contact fields when input that information. So far I know the following

Condition:

CUSTOMER_HEADER_BLK.CUST_ACCOUNT_NUMBER=

Action:

CUSTOMER_HEADER_BLK.CONT_PER_LAST_NAME_C=

CUSTOMER_HEADER_BLK.CONT_PER_FIRST_NAME_C=

CUSTOMER_HEADER_BLK.CONT_CONCAT_PHONE_NUMBER_C=

CUSTOMER_HEADER_BLK.CONT_EMAIL_C=

I have no clue how to tie these all together, so if I just input any account number it will pull up the contact information associated with that Account number.