primary keys

I would really like to know how I can define a primary key in postgres.
The database is for a search engine. The table looks like this:

CREATE TABLE Homepages (
    URL            varchar(80),
    PageName        varchar(80),
    UserName        varchar(40),
    Keywords        varchar(255)
);

And I'd like to define the attibute URL as the primary key but I've
got no clue on how to do that. Anyone?

Thanks in advance,

Perry Seleski
Rotterdam Art Institute