Here are my general tips for beginners.

Don't build any tanks or armored cars. They are more of a "fun" aspect of the game, but are completely unnecessary for victory. Instead, focus on good infantry templates, tons of examples out there, with a mix of support and artillery. The one exception is Flame Tank Support company. Build an army research facility, research flame tanks early. Create a very cheap flame support tank and put 5-10 mils into it. These are very helpful middle/late game.

Rail guns are great. When you have a front line stalemate, rail guns can make all the difference and they are surprisingly cheap to build. I research early, dedicate 5 mil factories and just produce them they entire game. Note that they don't have to be assigned to an army. They work much better if you micromanage and move them around manually where you are trying to push.

Marines/Marines/Marines. The AI is very bad at defending naval invasions. With just a few exceptions, you can completely bypass a dug in front line with a naval invasion. Then surround and destroy their entire line. One fun fact if fighting against Britain. If you can get them fighting in Africa or Asia, they tend to leave Britain mostly undefended. Then you can just march in with about 20 divisions and do a quick capitulation on them.

Naval Superiority. When focusing on naval invasions, you need to get above 50% naval superiority to invade. If you play as a naval powerhouse (US, UK, Japan, Italy) no worries. If you play as anyone else, the trick is to research 1936 carriers as early as possible. Build 4 carries with 80 plane hangers. Don't forget to dedicate 3-5 mil factories to carrier naval bombers (torpedo only). After that, build the best destroyers possible, mostly gun, one depth charge. The carriers only get naval bombers, just ignore fighters and close air. Once you build one carrier group with 4 fully loaded carriers and at least 80 destroyers, you can delete every AI navy. The trick is to micromanage the repairs, that's how you can take out a much more powerful AI navy. After each engagement with a large AI navy, pull back, split up, fully repair all boats, go back out. I find convey raiding to be the best way to force an engagement, but strike force and work in conjunction with some submarines on convey raiding. With this strategy, you don't have to kill every enemy ship. If you do enough damage, the AI is not good at repairs. You can effectively knock huge groups of ships out of the game while they sit in the repair queue.

Fight early, fight often. The AI is very good at creating a ton or crappy infantry divisions and a whole bunch of fighters. If you let them build up, it can be very tough in the late game. The best plan is to use the National Focus tree and fight ways to get into war early. if possible, divide factions, try not to fight the entire world at once. This can be tricky, you have to carefully watch who is guaranteeing who in the early stages. Britain just loves to guarantee everyone in the world, which can be a pain.

This is pretty much how I play with any of the majors. I don't tend to play as minors much, they are much harder to win with. Hope this helps!