Hello Francisco,
i hope you can understand by bad english.
There are two things in your Applet which are incorrect.
At first you have to download and install a JDBC Driver for your
postgres database (http://jdbc.postgresql.org). The install manual at
this side is for Java-Applications. That means that you have to
install the driver in the directory where your Java JDK is on your
harddisk (it's just a jar-File).
But in an Applet will be called from anyone from the internet with a
webbrowser. And in the JRE of the webrowser is no JDBC driver for
postgres. So you have to extract the jar-File of the JDBC driver to
your path of the harddisk where yours Applet class is.
I don't know if i'am right but i think your applet is not certified.
An applet has no access to the Network or to the Filesystem if it is
not certified.
If you want only test how a Database work with Java so you should take
an application. An application has access to the Network and so on.
There are no limits. But if you want an applet you have to sign it.
this is a not easy process at the beginning. If you want so, i can
give some advise to sign an applet.
Jan