Medium tanks, and Artillery.

Find a good alliance. You get better by playing with and against better players.

Keep playing learn what works and what doesn't work even if you are being defeated stick with it. Don't to use gold for just for the win learn the tactics of the game for both offence and defense'

Study your unit defensive and offensive capabilities.

ie: Inf are better on defense. if you see an enemy stack coming in, don't attack. Sit tight and defend taking in account terrain bonuses etc.
Learn what you need to defeat what is being thrown at you.
Artillery? Hit them with TAC bombers.

Simple logic.

If you are being overwhelmed with large numbers of units? Kiss your a** goodbye.

BMfox is a great YouTuber with tons of great tutorials available. He also has an awesome Discord community with experienced players happy to help. Call of War also has a good Discord server.

If you are hoping to learn on your own you need to understand which units are defensive and which are offensive, learn about stacking (i.e. attacking with 1 or 2 units will end poorly), and you need to know the basics of doctrines.

Take this wiki overview to understand your doctrine capabilities.

wiki.callofwar.com/wiki/DOCTRINES

This game has a LOT of tricks. But here are 4 easy things to do first.
1-Use the terrain always. Tanks and Armored cars on plains. Infantry in cities. AT guns in forests and cities. Artillery in hills. Knowing terrain is important.
2- Try to attack with 4-1 advantages.
3-Don't build everything in every city. You don't need a barracks, aircraft factory, port, tank plant, ordnance factory in EVERY city you own.
4-Stacks your troops in piles. He who defends everything defends nothing. Prioritize and hold your cities and good terrain spots.

There is so much more to learn, but these tips get you started.

P.S. Don't go crazy and research everything to level 2 either. Units take longer and cost more at higher levels.