Do not bunch up infantry. Never. Split the squad into smaller units, or preferably, micro every soldier into position.
I suggest you play competitive rules with normal resources. 50k spamming games wont get you anywhere. Play some singleplayer to learn the basics.
Bind commands to certain keyboard keys, do not click stuff, it wastes valuable time.
Always use sandbags. Do not place them all in one line, space them out so you can cover larger areas.
Learn and use direct control. It makes a big difference. Direct control fire the machinegunner early game.
Grenades = the game. Learn to cook them. You can toss them over building sometimes too. Destroy blobs with a single grenade. Throw AT grenades on the engine of a tank. Use smoke grenades. (They dont work perfectly, you must use a lot of them to actually cover anything).
Pick up stuff from the ground/dead bodies. Reuse sniper rifles. Always move your sniper after taking a shot. Learn to use the flamethrower, it is deadly in tight areas and when being fired from a foxhole. Use the holes in the ground as cover. (You can cheaply hold ground with militia this way. 1. Put the militia into the holes. 2. Laugh at the enemy wasting an artillery barrage to kill 2 militiamen).
Infantry wins the games, not tanks. Buying a Kingtiger and rushing it will not net you victory. Always use tanks together with infantry, otherwise the tank WILL die to a bazooka fired from a bush. Always get an AT rifle early game. Hide it in a bush and ait for a light vehicle to rush you, then destroy it. Wait for him to get closer or to show his side tho. If you have a heavy tank, or a lot of artillery, always hide a bazooka or an aa gun (Flak, PKP, oerikon etc.) near it, to prevent any light vehicle rushes.
Stick to "Combat" for starters, where you have enough time and do not need to rush. Play Assault Zones only after you know the basics. If you screw up the beginning of AZ, it will cost you the game (most likely).
Be more specific about what advice you seek, the game is too vast
Try and find someone to play it with, or to give you some ingame advice. A big part of this community are arseholes, but you might stumble across some nice guys too.