SQL & PL/SQL :: Create Trigger On (account) Table?

I gave this a shot on my own, and failed, so I am putting the question to you:

I am looking to create a trigger on the "Account" table

CREATE TABLE Account(Acct_no number, BillAmt number, OpenDate Date, SSN number,
Primary Key (Acct_no),
Foreign Key(SSN) references ResponsibleParty(SSN));

which will update the BillAmt field whenever one of the account's respective records is inserted/updated/deleted from the following tables:

CREATE TABLE Plan(Plan_id number, Cost number, Minutes number,
primary key(Plan_id));
CREATE TABLE Feature(Feature_id number, Description varchar(20), Cost number,
primary key(Feature_id));

These tables are linked through the following table:

CREATE TABLE Phone_Line(MTN number, cStart Date, cEnd Date,
rDate Date, Status char, ESN number, Acct_no number, Plan_id number,
primary key(MTN),
foreign key(ESN) references Equiptment(ESN), foreign key( Acct_no) references Account(Acct_no),
foreign key(Plan_id) references Plan(Plan_id));