What to do with no foreign keys

If I have two tables, which are like this:

baseball_team
-------------
team_name
number_of_players
games_played
total_runs


baseball_game
-------------
home_team_name
visitor_team_name
home_runs
visitor_runs



How can I insure that for example "home_team_name" is one of
"baseball_team.team_name"?

I have seen in books things like:

PRIMARY KEY: home_team
FOREIGN KEY: home_team JOIN TO baseball_team.team_name

is there anyway to do this in postgresql? Bascially I have like 12 tables,
and alot of them have a field "username".  I want to insure that
"username" in all tables, is a valid username in "passwd.username".

Brian


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