Beginner

If you are just starting out I’d recommend sticking with the infantry and artillery combo. General tactic is to flank with your infantry and bombard the enemies with artillery and then finish them with the flanking infantry. For offense you want to build riflemen to flank as they have the best movement for infantry and grenadiers to help assault cities, pair them with heavy/siege artillery as support to take down cities. For defese of cities after you take them, just use militia they have one of the best Hp to resource spent ratios. You only need to spawn them if there are enemies in range of capping your city.

If you go with this strategy, your early game skills should focus on upgrading light infantry and heavy artillery, preference on offense before defense upgrades. Mobility upgrade is top priority if it’s available.

For gens you could either go infantry or artillery first as long as they have the aura skill. Personally I’d recommend Massena first and then buy an ambulance item for him, the regen will carry you through early campaign, but it’s up to your playstyle preference.

You will naturally unlock princesses as you progress in campaign. There are also some unlocked through conquest achievements.

yoU aNd yEr cAv gEnEralsSs

One Naval commander will do, I personally choose Admiral Hood he is almost as strong as Nelson but much cheaper with the discount if you can pass the levels to get him. Navy becomes important later in the game when amphibious operations are most going to happen, if you do not have a good navy it becomes tedious to take on Britain or either Europe or America in conquest. 

Battleships are really good at destroying enemy fortifications and are overall the best, but try not to use them to solo enemy ships it drains the ships combat efficiency when damaged making them useless since ships do not heal.

Frigates are the main ships that you will have the most in your fleet, they should be used to protect your battleships and engage enemy warships.

Corvettes are not necessary unless you plan to take a weak coastal city but even that's a stretch, they would be better used to surround ships to get that sweet moral debuff on your enemy.

Clippers are useless don't get them unless you are desperate if you have a lot of them you could surround the enemy but they are too weak to take any sort of direct fire that the enemy would just break out anyway.

Overall the navy is their to protect and support the army, when the army has pushed back the enemy out of naval attack range it's best to disband half the navy.

Cav gens are super strong, but their main strength comes from being able to clear several weakened units off the map in a single turn while maintaining their hp levels high. You’ll need other gens to help soften the units up first before you send in the cav.

Of course, buy aura generals that can reach prince to have a level 5 skill and none of those weaker ones