The Basics - make sure your maintenance slider is maxed, that you have the newest unit types, that you're not taking too much attrition, and that your big stacks have leaders.
This is not a RTS, slow the game down, and pause a lot. I generally run at speed 2-3 during war, with frequent pauses. I have my game set up to automatically pause whenever i start or end a battle, or whenever a siege is completed. This helps keep track of everything going on.
Attacking is bad, especially early on. Defender bonuses are large, so in the early game try to force them to attack you.
Unless you have innovative + lots of tradition bonuses, the AI will tend to have better generals than you. They blow MP re-rolling them, and Lucky nations get better ones by default. You can still win, but keep this in mind.
Keep an eye on your manpower, if you run out, your units will not be reinforced. Merc's don't pull from your manpower pool, so they are very useful early on when manpower tends to be tight. You can also use the ledger to check your opponent's manpower. Unless your fighting Russa, the emperor, or the Ottomans as a non-sunni, running the enemy out of manpower is one of the best ways to defeat them.
Be aware of your neighbor's military tech. Every tech level is impactful, especially the ones that grant new unit types. I'm generally against buying tech ahead of time unless you have nowhere else to spend the points, but military tech is a big exception.
Also keep an eye on your morale, an army that has just fought will tend to have low morale, even if they've won.
Likewise, units fight by regiments, and depleted regiments do much worse. If we both have 20k troops, but mine is 20 full strength regiments, and your's is 40 half strength ones, mine will fight better. You can consolidate regiments, or fall back and let them recover. However, you cannot consolidate mercs, meaning that if time is of the essence and money is available it can sometimes be worth it to fire damaged merc regiments and hire new full ones.
Pre firearms, armies should be something like 60%/40% infantry/cavalry, if you can afford it. Once you have firearms, start to shift more toward infantry, and integrating artillery. The best late game army is 40 infantry (the max combat width) 40 artillery (which can fight from the second rank), and a small number of cavalry (to flank).
Speaking of combat width, keep it in mind in the early game. It's really not worth it to throw 40+ regiments into a fight if combat width is only 15. You'll just increase your attrition. However, If you have two separate "stacks," you can rotate them in and out of a fight, which can help you preserve morale and outlast your opponent. This is particularly powerful in mountains, or other areas where combat width is limited.
Now strategy and tactics
Keep your troops away from the border during peace. You'll likely not be at full maintenance, and if someone declares war on you and sends in a stack that has been at full maintenance for a few months, you will be crushed. Conversely, if you're being aggressive positioning troops to do this can be advantageous.
When attacking the AI will generally spread out to siege a few provinces while keeping 1-2 large stacks. Fall back, try to pick off sieging stacks, and encourage the "hunter killer" stacks to attack you, preferably into favorable terrain.
When you win a battle, you the enemy armies will run back to their territory, often to their capital. They can be pursued, but unless you have forced march they will move faster than you. Early on, or against an evenly matched opponent, this is extremely dangerous, but it can be an important part of late game strategy.
Speaking for forced march, when it's on your stack won't recover morale, even if they're just sitting still. So, don't leave it on unless your using it.
Always keep the war goal in mind, ticking warscore is a powerful thing.
As long as you are in you are in owned, friendly, or occupied territory, your generals can teleport from army to army instantly. Always be aware of where your generals are, and be ready to move them in response to enemy stack movements. For example, if you have a large stack with a general sieging the capital/war goal, and you see an enemy stack attacking elsewhere, you can detach all but 1 regiment, leave the new large force sieging, run the 1 regiment with the general back to friendly territory, and then move your general wherever he is needed.
It's fairly easy to box the AI in with "countermarching" If you move to attack a stack and the AI doesn't want to fight, they will wait and start moving away on the last day possible, so they will leave the province the day before you enter. Likewise, if they try to advance into an empty province and they see that you have a stack that will arrive their first, they will often cancel their move command. With two stacks, pausing, and micro, you can abuse these tendencies to do things like pin a stack in place while one of your armies moves to attack it, or draw them into ambushes by forcing them to move and having one of your stacks enter the province their moving to from outside their fog of war. This takes a lot of practice to master, but if you just fight at slow speeds, pause frequently, and pay attention you will learn all sorts of tricks here.
The surest way to win a war is to destroy the entire enemy army, and spread out and "carpet siege" every enemy province. Enemy units will come out one at a time with limited morale, and will be destroyed by your forces with very few casualties. This is hard to do early on, especially when fighting an alliance or a larger opponent. An easier option if you've destroyed the entire enemy army can be to siege several provinces while keeping a large concentrated stack moving around the territory you aren't sieging killing regiments as they spawn.
Quick review of military relevant ideas
Innovative is deceptively powerful, with tradition bonuses from other ideas and buildings, the reduced decay can leave you hovering around 70 in the late game. This will ensure you always have decent generals, but isn't that powerful early on.
Offensive and Defensive are both very powerful, and any land power should take both eventually. Offensive increases your manpower, force limits, and your troops lethality, and also unlocks forced march. Forced march is extremely powerful, it lets you run down armies you've beaten, and concentrate forces where they are needed. Defensive's bonuses are more subtle - increased morale, tradition, defensiveness, reduced attrition to you, and increased attrition to your opponents. I generally prefer offensive first - i highly value the increased manpower am force limits, and and pretty good at abusing Forced March. Still, you should get both, probably in your first 5 ideas.
Quality is never a priority, but very nice. The bonuses to each unit type are useful, and the discipline bonus at the end is crazy. It's doubly useful for naval powers, as it helps your ships as well. Never a priority, but a solid late game idea if you don't have anything better.
Aristocratic sucks, and quantity isn't much better.
Plutocratic is awesome, but not so much for the military bonuses. Take it if you're a republic with access to it, but don't count on it to help you much in a fight.
Hope this helps. I'm kind of just listing stuff off here, and I'm sure there's more, but this should be a good start.