SQL & PL/SQL :: Function - Add Tax To Particular Order In Database?

I created a function and what I'm trying to do with this function is add tax to a particular order in my database. I want it to multiply the quantity and cost of the order then add tax and then insert it into the corresponding column.

for the tax I'm using this function:
Create or replace function tax(v_value in float) return float is
Begin
Return(v_value*.07);
End tax;

select PRODUCT_ID,TOTAL_COST, tax('TOTAL_COST') from PRODUCT;

I get an error saying TOTAL_COST is an invalid identifier.

This is my table structure:
SQL> select *
2 FROM user_cons_columns
3 WHERE table_name = 'CUSTOMER';

OWNER CONSTRAINT_NAME

[code]....

I want the total cost to be inserted into the total cost column.how I work this out through either a function or procedure or both?