Below are several tips to help new players.
Grab Town Centers As Fast As Possible
In all games, with the exception of some campaign levels, town centers are key to producing more units. Train fast units to explore and take over town centers before the enemy takes them. Scouts cannot take over town centers. Train defensive units to protect your newly conquered towns from enemy exploration units.
Start Building immediately
Buildings are very important. If an attack is expected, build walls or defensive towers. If enemies are not nearby, build troop generation buildings such as barracks to help generate an army.
Build A Mixed Army
Creating an army out of a single unit type will result in that army being eliminated by an enemy mixed army. Use a combination of defensive/melee units, ranged units, and healing units to fight against enemy armies. Train a mix of spearmen and swordsman, as they have different strengths and weaknesses
Pick Battles Wisely
Units are weak to some and strong against others. Attack enemy units with units that are strong against them.
Archers [Archer]: Weak to cavalry, skirmishers and heavy infantry; Strong against swordsmen and flying
Skirmishers [Skirmisher]: Weak to only cavalry particularly, but low hp; Strong against archers
Heavy Crossbowmen [Pavaise Crossbowman]: Weak to cavalry, heavy infantry, and skirmishers; Strong against swordsmen, flying, heavy infantry, and heavy cavalry
Heavy Cavalry [Knight]: Weak to pikemen, heavy crossbowmen; Strong against swordsmen, archers, skirmishers, and heavy infantry
Light Cavalry [Light Cavalry]: Weak to pikemen; Strong against swordsmen, archers, skirmishers, and heavy infantry
Pikemen [Spearman]: Weak to none specifically (power range units?), but low hp; Strong against cavalry
Swordsmen [Swordsman]: Weak to archers and cavalry; Strong against structures, siege machines, and ships
Shielders [Shielder]: Weak to cavalry; Strong against structures, siege machines, and ships
Heavy Infantry [Foot Knight]: Weak to cavalry; Strong against structures, siege machines, ships, swordsmen, and archers
Siege Machines [Catapult]: Weak to melee units; Strong against structures and ships
Attack Ships [Galley]: Weak to anti-ship ships, siege machines, swordsmen; Strong against transport ships
AntI-Ship Ships [Trireme]: Weak to siege machines, swordsmen; Strong against ships, siege machines, and structures
Flying [Ornithopter]: Weak to ranged units (especially archers); Strong against melee units
Avoid engaging stronger enemy units with weaker units in melee (ranged attacks have no risk of retaliation in most cases). If you have to (as in some campaign maps), try attacking first and minimising damage received.
Defend Your Support Units, While Attacking Your Enemy's
Builders, Healers, and Archers are generally low heath. Eliminating enemy builders prevent your opponent from building structures that can swing the battle. Eliminating enemy healers prevents your enemy's army form recovering health.
Utilize Choke Points
A narrow area, such as a bridge, or mountain pass can slow the progression of a large army. This will can give you time to train, move, or heal units as well as pelting the enemy army with projectiles.
Use Waypoints to save time
When the battle is not directly near your training areas, migrating newly trained units can take some time. Waypoints can automate the repetative process of moving units. Set waypoints to bring units to the general area of a battle.
Try to research technologies if you have spare capacity or in a large map. They will pay off in the end. A Blacksmith or Advancements Centre is very useful. You can delete them later to free up space for training units.
Logistics are the most important part of a battle. Try building roads, bridges, transports, wagons, and factories (especially mines) when you have the time.