My tips for the campaign map is just to take things slowly. Get a good stack of 15 - 20 under your starting leader and take and hold your starting region. Building walls in your settlements is a good priority because the AI loves to come in and snipe unwalled towns, burning down any advanced buildings you have painstakingly built up. Once you have a secure home region bringing in a stable income, get a second army stack going and choose a neighbour to take over. Rinse, repeat.
For battles, this will be your battle setup 99% of the time.
Put your archers (no more than 4 units) out the front to hit the enemy as they approach. Make sure you set them to ‘skirmish mode’ so they will automatically fall back behind your main line as the lines close together.
Then just use good ol’ Hammer and Anvil tactics. Your main line will be spear and shield infantry (say 8 units). They don’t do much damage, but are good at defence and will hold the enemy in place (the anvil).
Your cavalry on the wings (4 units) can then swing around from the sides and hit the enemy line in the rear (the hammer) - all units in Total War games hate being attacked from the rear and will break quickly once this happens.
That’s 16 units thus far. The other 4 units are a mix of your hero, maybe a caster, plus a couple of special monster unit to act as a shock force in reserve.
That’s a very basic plan. Once you get into it you can look into more advanced things like Armour Piercing values and anti-large values to tweak your army depending on who you’re fighting.