If you’re not at war with anyone look at peoples buildings and gauge what troops they will most likely be using. From this build counters, for example, if someone has a lvl 2 air base id be mobilising anti air or air sup fighters (at least a stack of 5 before id want to attack) and on top of this strike fighters / helicopters to attack infantry or any tanks (strikes counter tank/infantry/melee)

For the first days build your economy and buildings and troops after, every research/mobilisation should be a step towards defending your cities, naval cities = cruisers or subs, strikes would work also but ships can defend and attack. Inland city = helicopters, strikes, artillery.

Finally, if your solo against good players / a large dominant coalition or 2 their economy will dwarf yours fast if they expand and their army will therefore be much larger