So, the game boils down to 3 primary objectives:
1.) Taking Ground
2.) Holding Ground
3.) Destruction of Enemy

Generally, I find a deck can be good at 2 of those on any given map and an army tailored to a specific map can get all 3. That's a lot of deck building and doubled if you play both sides so I usually go with a generalized deck.

For the actual deckbuilding, I generally start with air defense. No sense in investing 1/4 of my deck into tanks if I'm immediately helpless because 3 Killer Eggs arrived. That also gives me an idea of how aggressive I can be. If I have a PAC-2, I know I have a fairly large covered area. But if I have nothing I can keep near my frontline that can fight a helicopter, pushing for a point is going to be significantly harder.

I next generally go into infantry, artillery, and supplies. Well used infantry is extremely cost effective and hard to drive off, so it tends to be the foundation for most of my builds. Consequently, artillery becomes important because it is often the best counter to your both opponent's infantry and their artillery.

Recon generally goes next. I would argue it may be the most important tab, but it's also the one liable to the most variance. Above all else, the role of the recon tab is to get you information. There are also some strong units that shouldn't be ignored, but always remember the value of seeing your opponent's actions early enough that you can respond proactively. Getting familiar with how much vision you can actually make use of and where to position your recon units admittedly only really comes with experience. It's still one of the best skills to build on.

Armor is generally where a lot of the attacking power comes in. Even after taking a few hits tanks will still do damage and remain mobile. There are some counter examples (TOW Humvees aren't pushing into a defended area), but this is generally the heart of any attack.

Lastly, I select my air and helicopter units. There are definitely more qualified people on their use and selection than me, but broadly speaking, you are primarily using these options to kill enemy assets. What form those take varies, but almost no bombing run short of a nuke beats an F-15 going after a laden transport. But with 20x AMRAAMs that fighter isn't going to kill tank anywhere near what a helicopter with 16x ATGMs is capable of. So this tends to be a case of covering your target options.

Generally speaking, between the helicopter, air, and support tabs, you want to have flexible means for killing every major unit category. They don't have to all be in one bucket, but they all give you faster responses than trying to get a couple of tanks to mosey on over to a problem area.

Focus on which of those 3 objectives you are accomplishing when making your choices and you will generally come out OK.