scripts in Postgres

Hi

I am fairly new to Postgres and am struggling to understand one concept. If
I wish to use pl/pgsql, must it be in a function?
An example of where I would not want it to be in a function is:

I have a CREATE TABLE statement that I want to execute. But the script must
first check for the existence of the table. I wrote the following code, but
it errors on the first word (IF). Please tell me how to do this:

IF EXISTS (SELECT * FROM information_schema.tables WHERE table_Name =
'tb_Messages')
DROP TABLE tb_Messages;

CREATE TABLE tb_Messages (
MessageID bigserial PRIMARY KEY,
From varchar(255),
To varchar(255),
DateSent timestamp not null DEFAULT current_timestamp,
Subject varchar(255) NULL,
MessageBody Text null,
IsRead smallint default 0,
DeleteFlag smallint default 0,
AdditionalInfo int NULL,
ReplyToMessage bigint NULL

) WITHOUT OIDS;

Thanks
Craig