Sadly, it is easier said than done. There may be other websites that can make this easier, but this is how i have been doing it for many years. Your prolly not gonna understand a single word i say, but if you get lost in the readme's this will help you out.
For starters, your game must be "allowed" to have modifications. Metamod is the tool needed to do that.
www.metamod.org . Follow the complete instructions. You are basically adding a new folder to your 'dod' server folder called addons/metamod/dlls the file that goes in the folder 'dlls' will be something like metamod1_8.dll (thats not the name).
Find your liblist.gam and edit the second line from the bottom which points to a dayofdefeat.dll. change that line to direct it to your metamod.dll .
Now your ready for the actual mods.
Theres adminmod and amxmod and now amxmodx. go to
www.amxmod.net for the download and complete installation instructions. what your doing is basically adding another folder to your "addons" called "amxmodx". when thats all done, you will have to download statsme from the plugin section at amxmod.net add it to your custom plugins folder, and edit the plugins.ini file in addons/amxmodx/config . Make sure you read up on the readme's for the statsme plugin, so you will know the rcon commands in the game to configure the sounds.
I dont expect you to understand this. But its more of an eye opener of to how complicated it can be to add a simple modifaction. My best advice to you is go to
www.amxmod.net and
www.metamod.net and read all the readme's and learn as much as you can. If your paying for your server hosting, contact your host to set it up for you, if your running your server of your machine, you might want to consider what your getting yourself into
.