Ah, alrighty then, you've got the plugin - thats a good start chap, don't lose hope just yet! BTW, bit of *nix that ppl rarely mention: to run something from the CLI from the directory in which it resides, you need to put a ./ in front, like
[root@localhost libexec]# ./check_http
check_http: Could not parse arguments
Usage: check_http -H <vhost> | -I <IP-address> -u <uri>] -p <port>]
-w <warn time>] -c <critical time>] -t <timeout>] -L]
-a auth] -f <ok | warn | critcal | follow>] -e <expect>]
-s string] -l] -r <regex> | -R <case-insensitive regex>] -P string]
-m <min_pg_size>:<max_pg_size>] -4|-6] -N] -M <age>] -A string]
-k string] -S] -C <age>] -T <content-type>]
[root@localhost libexec]#
If you run it with the full path, it isn't neccessary (as you have found)
From here on in, to monitor your HTTP service it's pretty much as described in nagios.sourceforge.net/docs/3_0/ ... vices.html
To keep things nice and simple I suggest you create a 'httptest.cfg' in your nagios/etc/objects directory and put the bits and bobs in there. So what you need to do first is create your 'host object' which, at it's simplest will look something like
define host{
use generic-host ; Inherit default values from a template
host_name webserver ; The name we're giving to this host
alias Some Web Server ; A longer name associated with the host
address 192.168.1.50 ; IP address of the host
}
Then, you need to define your service object, like
[blockquote]define service{
use generic-service ; Inherit default values from a template
host_name webserver
service_description HTTP
check_command check_http!-u someserver.com/index.html
}[/blockquote]
*Replacing the -u http...with the url of the web page you wish to check that is being served from the host server as you would put it in the address bar of your browser.
The ! symbol in the check_command line delimits your arguments as they appear for when they are passed through to the check command. What happens is, if you look in commands.cfg you will see a command object definition for check_http that finishes with $ARG1$. This is replaced by whatever comes after the !... As we only have the one argument to satisfy, anything after the ! in the service definition becomes $ARG1$, therefore if your webpage has some basic authentication on it you can change it to "check_http!-u someserver.com/index.html -a username:Password" for example... Other checks may have multiple $ARGs in the command definition and you will no doubt become familiar with them in the future. When you define your service checks for these, you just use multiple !s, like "check_whatever!somearg!someotherarg!yetanother!etc"... anyway... I digress.
Next, make sure the permissions and ownership are right on the httptest.cfg file. To start with they might look like...
[root@localhost objects]# ll httptest.cfg
-rw-r--r-- 1 root root 0 Aug 11 18:12 httptest.cfg
Change the ownership...
[root@localhost objects]# chown nagios:nagios httptest.cfg
[root@localhost objects]# ll httptest.cfg
-rw-r--r-- 1 nagios nagios 0 Aug 11 18:12 httptest.cfg
*or use whatever user:group you set up for nagios if they are not that
Change the permissions
[root@localhost objects]# chmod 664 httptest.cfg
[root@localhost objects]# ll httptest.cfg
-rw-rw-r-- 1 nagios nagios 0 Aug 11 18:12 httptest.cfg
*More *nix... the 664 relates to the three groups of rwx (read, write, execute) permissions on the file. It's like binary, the r has a value of 4, the w is 2 and the x is 1, so as we want r and w for the first and second set of permissions, thats 4+2=6, and for just the r for the 3rd set, well that's just 4, so thats what the 664 bit means. When you create your own checks (!) in the future you will need to make them executable like the other checks in the libexec directory, so that's probably worth knowing 
Happy days. Lastly, modify nagios/etc/nagios.cfg and add
cfg_file=/usr/local/nagios/etc/objects/oncall-group.cfg
Just stick that line in somewhere near the other cfg_file statements.
Lastly use the -v switch to do the 'pre-flight check' and make sure everything is in order, and with any luck you're free of errors and you can restart nagios and be on your way! If you are having errors on the pre-flight check, do post them up and we'll see what's what (i.e. fix any stuff I just forgot to mention)
One thing of course, it's a lifesaver to make a copy of anything you are going to change, just incase it goes south and nagios won't restart, then backing out your changes is just a matter of copying the original file back - always handy 
HTH
/S
