To enable the use of Pixelmon you are required to run a Modded Branch of Minecraft on v1.12.2.
To do this you will need to install Minecraft Forge - MC 1.12.2 - 184.108.40.20655 which can be downloaded below (Make sure to wait the 5 seconds and then click 'Skip' in the top right, DO NOT click any ads):
You will want to download the 'Installer' version which is a lot more user friendly to install, once that has downloaded open the file and make sure the option 'Install Client' is selected and not any others or it will not work and if you have installed Minecraft to a different location than default you will want to locate where you installed it, otherwise leave the File Path as normal.
While Forge is installing, now would be a great time to start downloading Pixelmon which can be downloaded below:
Once Pixelmon has finished downloading and Forge has finished installing you will want to locate your Minecraft files, which if you installed Minecraft to the default location its a breeze otherwise find where you installed your Minecraft, to find the Default Location on Windows 10 you can press 'Windows' key and type %appdata% and then press enter, if on any older version of Windows you may need to press the 'Windows' key + R to open the Run program, then type in the field %appdata% and then press 'Ok' or 'Enter' you should now see '.minecraft' right up the top, open that folder and if Forge was successfully installed you will see a 'mods' folder, this is where you want to place the newly downloaded Pixelmon Mod File, this too is where you can install other mods accepted by various servers such as JourneyMap etc which is completely optional.
Once you have atleast the Pixelmon Mod inside the Minecraft mods folder, you may now Start up Minecraft (or restart it if you left it open), once the first Minecraft window has opened, if you look at your current version, if it says Latest release - 1.17.1 you are NOT on the correct version, you will need to click the dropdown box and find one called 'forge' - 1.12.2-forge-220.127.116.1155 if that is already selected you can simply press 'Play' and you should now notice a 'Mojang' Loading screen that is initializing the newly installed Mods, this may take some time the first time you do this.
If you are presented with the Minecraft Starting Screen with * mods loaded in the bottom left that means everything should be installed successfully and you can now join Pixelmon Reforged Servers.