stick with vanilla for a bit, get used to the command map style play and the battle map.
Command map tips:
If you don't have an economy, you can't wage war.
Battle Map Basics:
Swords beat spears
Spears beat horses.
Horse charges beat swords.
Archers are good against all, but die in close combat/run out of ammo.
Horses are best in charges, don't let them get swamped into a standing fight.
Hit flanks, never charge a spear/pike wall with anything, higher ground tires enemy as they scale it and improves visibility and archer range.
charging downhill gives benefits.
Don't get flanked.
Thats enough basics to get the ball rolling, there are a whole slew of advanced tactics and strategies to employ and once you've tried some basic battles and got a feel for units you can look into these.
General overhaul mods that improve the game generally limit your ability to churn out huge armies late game (usually mid to late game you begin to snowball into an unstoppable machine)
They limit this by making recruitment pull from your population, and so you can't recruit if your population reaches a minimum level. This forces you into smaller numbers of armies and makes you MUCH more tactical about the battle map as you fight tooth and nail to win with the least losses possible, which isnt an issue in vanilla as you can just keep producing more people to replace losses no problem.