First and foremost, we are lacking critical information as:
How much industry do you have? are you able to afford a few medium tanks or mechanized infantry?
Whats the core of the enemy forces? Infantry with a couple of artillery and light tanks battalions? motorized infantry? mechanized infantry?
Do you expect to be attacked in one or several fronts?
Its there any choke point than you could exploit?
How much manpower you can use without crippling your industry?
Without taking that as consideration, then:
Start making OHQ battalions to counter the enemy army
Move your troops to the choke points or better defense positions (try to take advantage of nature)
If possible use militias on other flanks where you might be attacked but you're not sure
If you can afford to make artillery/tanks, make sure to make Siege Grenadiers formations or Heavy Siege if the enemy uses a lot of infantry
if the enemy its going to use lots of tanks, make sure to make assault guns with long barrels, they're not super good as medium tanks but they're cheaper and better at defense
if the enemy its going to use lots of tanks and you can't afford to make assault guns, then make lots of Heavy Rpgs OHQs, if possible use Heavy RPG or Advanced Heavy RPG
Make sure to protect your supply lines and your mines/industry, if they're not going to be in immediate danger, use militias to secure their surroundings
If you're sure than the war its going to last a long time and you can afford it, make "deep core mines" (Deep soil filtration IIRC), they are quite expensive at first but at level 2-3 they provide a good amount of metal and rare metals infinitely, so they're a very valuable resource in late game
If you're going to use lots of fuel, make sure to make hydroponics and industry to convert food to oil (I don't remember the asset name or tech) to have a secure oil source later on
If you're low on manpower, reduce the production of assets than you don't really need (maybe you have lot of energy without any use, or oil, or food)
Make sure than your supply lines are secure from either minors or your future enemy
if possible increase the truck station level at the capital, upgrade the supply depot level (the one towards the frontlines) to increase its range and change the roads to paved road, rail stations are vulnerable to strategic warfare (rockets and missiles) and since you will be fighting defensively you shouldn't need the big boost of the railway
If your capital and other important zones (mining/industrial hubs) are separated by mountains or long distances, consider making a 2nd SHQ, just bear in mind than the new SHQ should be self maintained, meaning than it needs its own food, water, electricity, IP, metal and rare metals production
You always need good leaders for the councils, but you also need good commanders to win the war, make sure than first and foremost they're of the same ideology than your government, a commander with bad relation will perform bad on recovering and motivating the troops.
All commanders should have at least 30 in war and 25 in Charisma, if possible they should be cap III, otherwise consider using them for covering fronts where you're not in war against majors.
If an enemy player its going to attack you, its because its confident in its bigger numbers, industry or tech advances, therefore you shouldn't expect to win the war since the beginning, you must first make sure than they don't breakthrough your frontlines and plan from there.
Its possible to beat an enemy with superior numbers, with techs or more resources, if you don't have any of this, try convincing another player to attack your common enemy so he his distracted and isn't able to go all in against you.
If the enemy its able to breakthrough, retreat just the enough hexes and make another frontline, otherwise the enemy its going to pocket your army and thats going to be your defeat.
Sometimes its not about winning the war, but making the enemy pay for every hex than he conquers, the longer he takes to conquer you, the more time other players will have to ready up and beat him later
I hope this tips helps you and good luck on your war :)
If you have further questions about the game or how to play in mp, consider joining the unofficial discord if you haven't