Welcome to the forums Philip Regent. Unfortunately, the ladder finished after one season and hasn't been kicked back into life, mainly due to both EoJ and me being a touch short on time to administer and promote it.
If you (or anyone else) would like to take on the mantle and relaunch the ladder then EoJ would, I'm sure, be willing to discuss what's needed to run it again and give any help required, post news as required etc. This is where lots of people rushing in and showing their interest in participating / support / volunteering would be excellent, but rather unlikely...
The general requirements to participate, if it launches again, will be simply to have access to online play (e.g. a game with valid CD key, Gamespyid.com registration and to be up to date with patches) and to be available for games on a reasonable basis. The last iteration was relatively informal without any player being required to play the person above/below them, making challenges / responsiveness less important than some types of ladder. Of course running scared from an opponent would never be supported.
I don't recall if there were strict limits last time over the use of mods or Kingdoms - anyone restarting the ladder could set such rules. If you've only got the main game have only played at a friend's to date, there are various places to purchase the base game and the Kingdoms expansion, either separately or as a Gold edition containing both. For the relatively small outlay for Kingdoms (less than £7, probably similar for the Gold version including the whole game e.g.
here) you get access to a lot more mods stretching from simple tweaks as far as community built total conversions like Third Age Total War.