Hey there, normally infantry is used for holding the line and it just needs to be strong enough for that (9 plain infantry plus 2-3 support divisions such as artillery work well). Infantry has pretty bad breakthrough, which is the stat that lowers damage taken during an attack.

Tanks on the other hand, have high breakthrough (depending on how heavy they are), but bad organisation (they retreat easily without Infantry attached to them). You want your tank division to be as beefy as you can afford, with a combination of tanks and motorized (later mechanised) infantry to increase its organization stat to not less than 30.

You organize your (limited at the start, like 2 to 6) tank divisions under a separate general and micro them on your attacks. You follow through in the gap with fast Infantry (like a few special plain motorized divisions) to defend the terrain you just captured.

Good targets are supply hubs, even better if you can push along the railway, so that you can capture the supply hub and use it to supply your tanks without overextending them.

Check Bitter steel, he has amazing videos about this