I installed PostgreSQL on a computer with Mac OS X using the One click installer. Then I try to access PostgreSQL using the psql command, but it doesn't seem to be available.

I get this message:

psql
-bash: psql: command not found


Do I have to install something more? Or how can I configure PostgreSQL so I can use it on my computer?

Appears "the way" to install the client, if you want to use hombrew, is:
brew install postgresql

then psql (the client command line) will now be available to you (it also installs a local Postgres server/database, but you don't have to use that if all you want is the client).
Apparently there's also a 'wrapper' to the psql command to make it more "user friendly" also available via homebrew (brew install pgcli) in case interesting.  Pgcli is a command line interface for Postgres with auto-completion and syntax highlighting.
Another option is to install the libpq homebrew package, but it doesn't get in your PATH.
brew install libpq

So follow the instructions it says after install out to add it to your PATH, or brew link --force libpq or add a symlink.
