Games

my general advice (which works well in single player against AI) is to memorize some division and equipment templates and use them while you're learning the rest of the game. search the country you're playing to see what they are good at, and produce that type of division based on this guide which also has good explanations about the combat system. also always remember: land > air > sea, and negative modifiers hurt more than positive modifiers help.

bonus tips: it's safe to ignore naval combat. most naval tech is garbage but research submarines (especially snorkels when the time comes). make task forces with 10 submarines in them and do convoy raiding. don't use ships for actually fighting, instead use cheap naval bombers to kill enemy ships (AI can never figure out good anti air). after you've sunk most of the enemy fleet, use your pre existing fleet for gaining naval dominance for naval invasions, and if you don't have a pre existing fleet your country is not a naval country so just ignore it.

for air, your troops are always number one priority, therefore first priority must be maintaining air superiority with fighters for the buff (and not having your planes shot down) and then CAS to damage the enemy. tactical bombers are not worth it in my opinion compared to CAS and strategic bombers are too expensive for large deployments.

if you have any specific questions i would be happy to answer.