You can use apt-get to install these packages by running the following commands:
sudo apt-get install apache2
sudo apt-get install build-essential
sudo apt-get install libgd2-dev
Become the root user.
Create a new nagios user account and give it a password.
On Ubuntu server edition (6.01 and possible newer versions), you will need to also add a nagios group (it's not created by default). You should be able to skip this step on desktop editions of Ubuntu.
/usr/sbin/usermod -G nagios nagios
Create a new nagcmd group for allowing external commands to be submitted through the web interface. Add both the nagios user and the apache user to the group.
/usr/sbin/usermod -G nagcmd nagios
/usr/sbin/usermod -G nagcmd www-data
Create a directory for storing the downloads.
Download the source code tarballs of both Nagios and the Nagios plugins (visit http://www.nagios.org/download/ for links to the latest versions). At the time of writing, the latest versions of Nagios and the Nagios plugins were 3.0b3 and 1.4.7, respectively.
Extract the Nagios source code tarball.
tar xzf nagios-3.0b3.tar.gz
Run the Nagios configure script, passing the name of the group you created earlier like so:
Compile the Nagios source code.
Install binaries, init script, sample config files and set permissions on the external command directory.
Sample configuration files have now been installed in the /usr/local/nagios/etc directory. These sample files should work fine for getting started with Nagios. You'll need to make just one change before you proceed...
Edit the /usr/local/nagios/etc/objects/contacts.cfg config file with your favorite editor and change the email address associated with the nagiosadmin contact definition to the address you'd like to use for receiving alerts.
Install the Nagios web config file in the Apache conf.d directory.
Create a nagiosadmin account for logging into the Nagios web interface. Remember the password you assign to this account - you'll need it later.
htpasswd -c /usr/local/nagios/etc/htpasswd.users nagiosadmin
Restart Apache to make the new settings take effect.
Extract the Nagios plugins source code tarball.
tar xzf nagios-plugins-1.4.7.tar.gz
Compile and install the plugins.
./configure --with-nagios-user=nagios --with-nagios-group=nagios
Configure Nagios to automatically start when the system boots.
ln -s /etc/init.d/nagios /etc/rcS.d/S99nagios
Verify the sample Nagios configuration files.
/usr/local/nagios/bin/nagios -v /usr/local/nagios/etc/nagios.cfg
If there are no errors, start Nagios.
You should now be able to access the Nagios web interface at the URL below. You'll be prompted for the username (nagiosadmin) and password you specified earlier.
The main page link is: http://localhost/nagios