So are you on Mortal Empires or Vortex? And what difficulties?
I’m going to assume Mortal Empires because of what you’re describing. I play on VH/N myself on both, and have played those starting areas (it’s a bit more complicated on Vortex). The main thing you want to do, is be extremely fast and efficient. You have to think a turn or three ahead. Like, if I fight this army, then what happens, and what happens after that? You don’t necessarily need a defensive army sitting in your own capital.
Both of those factions are tough starts in their own way. Skrolk is in a “mosh pit” situation, and doesn’t have great ally potential. Kroq is backed up against a wall, and surrounded by potential friends, who you’re actually going to want to kill.
Normally the first step is find anyone who you aren’t going to want to kill soon, and get a non-aggression pact with them. The computer hates to break these. Get trading and army movement if they’ll give them, but don’t do any kind of alliance.
That’s not so much the case here.
So next step, you want to strike as hard and as fast as you possibly can. Try not to have wasted turns. Try to make sure every turn, every movement of your armies, is bringing you closer to your actual goal, because the best way to deal with these guys is to kill them early, preferably very early, and like every turn or two you should be taking one of their cities, in some cases. Think about movement modes with Skaven - you quite often want to go under stuff with the teleport, rather than slowly walking.
Basically your opening move should usually be to hit the enemy army the starting scenario has put in front of you. You should have a way to defeat it with very low losses. Bring the hero into your army for the battle.
Then hit the town they were near - if you can hit it same turn, great, normally it’s next turn. If you can’t hit it same turn, and you’re not on friendly territory, think about retreating on to friendly territory so you can heal. Either way, end your turn on friendly territory, and build units. It’s fine if you basically zero-out your gold gain. But build the best units power/cost allows. With Kroq that’s Saurus Warriors with Shields (not spears and shields). If you can’t get those, get just Saurus Warriors.
This is basic stuff but these first few turns can be absolutely the difference between smashing the enemy, and having a real meatgrinder on your hands. Maintaining momentum is everything.
Building-wise, only build things which will help you. Don’t be just building because there’s a slot and a building is cheap or whatever. The exact priority varies a bit, but generally growth and money buildings (esp. efficient money buildings) are very good choices, and upgrading whatever building gives your best units (or some kind of units you can exploit, early-game) is what you want to do. Growth is kind of amazing particularly though. Public order as necessary (hopefully not too necessary).
Keep your eye on the prize. I can’t stress this enough. You gotta think about where you’re going. You want to fight the enemy lord in the open field, and kill them. That’s ideal. If you can see a way to make that happen, great. If the enemy lord get killed fighting someone else, or wonders off, bee-line their capital.
Use Ambush stance like a horrible bastard. When you use it, the computer doesn’t know where you are unless it’s fairly close and makes a check roll. You can use this to convince the enemy lord that it’s safe to fuck off, when in fact you’re moving towards his capital. Enemy lords are very good at predicting you’re about to attack them, so unless you use tricks, they will likely go to their capital to defend it. They also have near-perfect judgement on movement distances, so will know exactly how far you and they can move. But they have to follow the rules - if they can’t see you, they act as if you aren’t there.
If you’re not the Dwarfs, it can easily be worth losing a minor settlement to lure the enemy lord out of their capital. I’ve had to do this as Skaven a bunch of times. You don’t have to defend everything (though it sucks more for Dwarfs, Bretonnians, and Alarielle if you don’t). If you have an 14-unit army or the like, and they’re not total fodder, you can probably take a capital near the beginning of the game.
Be aware of whether you need to siege. Many units allow you to instantly attack a wall city. Monsters, a lot of lords (particularly big guys but also people like Vlad) let you do this. Otherwise you need to spend at least one turn (sometimes two) building a ram to qualify to attack the city. This can obviously be a problem if the enemy lord isn’t very far away.
In many ways the most ideal situation is if you or someone else just defeated the enemy lord in the field (hopefully in your own territory so you can heal up), and then can lunge towards their capital before they can field any kind of actual defensive army.
I usually spend most of the early turns (unless I’m flush with cash) with only one army and use positioning and knowing where the enemy is and what they’re doing and going to do to defend my territory. With Kroq you want to hit Khalida hard and fast and just keep taking her cities, and hopefully hit her main army before you get to her capital. Try and have like an army of bunch of Saurus Warriors (Shields) if you can. Really avoid Skinks for the most part with him.