Trigger Corresponding To A Sequence

I have 3 tables whose structure is as follows:

1) Invoice
Invoice_no number(8),
Debtor varchar2(35),
Invoice Date Date,
Print Date Date,
Currency varchar2(3),
Total Amount Ex Number(11,2),
Total Amount Vat Number(11,2),
Total Amount Number(11,2),
Status Varchar2(15),

2) Invoice Line

Invoice_no number(8),
Invoice_Line_No number(8),
Description_1 varchar2(35),
Description_2 varchar2(35),
Line_Quantity Number(11,2),
Currency varchar2(3),
Line_Amount Number(11,2),
Vat Code number(1),
Status Varchar2(15),

3) vat code
vat code number(1),
Description varchar2(35),
Vat Percentage number(11,2)

Here invoice no and invoice line no are automatically generated fields?

How to generate invoice line no corresponding to invoice no and how to satisfy the following conditions:

1) total vat amount in invoice table should be automatically filled . total vat amount=(line_quantity* line_amount*vat percentage)/100

2) invoice cannot be printed, changed when status is 'closed'.

3) invoice lines cannot be added,changed or removed when status of invoice is closed

4) when any of invoice line no is changed,added, removed or invoice is changed the status of invoice should be changed to 'new'.

5) invoice date of a new invoice cannot be of the past