How do I automatically update a timestamp in PostgreSQL

I want the code to be able to automatically update the time stamp when a new row is inserted as I can do in MySQL using CURRENT_TIMESTAMP.

How will I be able to achieve this in PostgreSQL?

CREATE TABLE users (
    id serial not null,
    firstname varchar(100),
    middlename varchar(100),
    lastname varchar(100),
    email varchar(200),
    timestamp timestamp
)