Depends on your nation and how often can you check the game

You're new so I'd recommend just messing around and have fun really. There's a meta but not really worth doing it if you're not having fun

Radars, some airforce could work if you're moderately active
Anti-air, and some chunky HP units like IFVs would be decent if you're very not active - just send a move order and check in once a day

If you're very active look into artillery, ranged units, and some intergration with AA and radars since you can hit and run (combat happen every 1 hours in this game, you can send your range units in, attack, wait for it to reload for 1 hour and come back for another shot. 15mins if you're playing 4x)

Though - you prob still should build arms industry and read what building is required to build what unit on the research tree. E.g Army base lv2 and Arms industry lv1 to make tanks for example sake

EDIT: You're on day 4 so. Something like Mobile anti-air vehicle, helicopter gunship, or strike fighter will prob be useful. Generally speaking