No serial type

Hi,
I've just started using PostgreSQL and have been reading up on it.
Part of what I wanted to do was to have a column which automatically
incremented itself by one every time I do an INSERT. From reading the
manual I was lead to believe that the correct type for this was either
serial or bigserial. But whenever I try and update my column to this
type it says that it can not find that data type. Am I doing something
wrong?

This is what I get:

simon=> ALTER TABLE users ALTER COLUMN userid TYPE serial;
ERROR:  type "serial" does not exist

I'm running this version:

simon=> SELECT version();
                                          version
-----------------------------------------------------------------------------------------
  PostgreSQL 8.3.3 on i386-unknown-openbsd4.4, compiled by GCC cc
(GCC) 3.3.5 (propolice)
(1 row)

Thank you or any help.

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