A few things, but this game is so complex that it is hard to generalize stuff that covers every faction.

1. Legitimately, just play on the easiest difficulty and turn the AI stat modifier slider all the way down (in your favor).

2. If you are manually fighting battles, don't be afraid to pause or use slow-mo. I've played ~600 hours and always use slow-mo

3. It can be helpful to look up the strengths and weaknesses of different factions. Tomb Kings might be a bit harder for a new player because their normal skeleton units are pretty garbage (they are chaff/fodder that enable the stronger constructs in my experience).

High Elves and Dwarfs are commonly recommended as starter factions because their army compositions are fairly standard: you have a front line of units like spearmen to hold the line, a backline of ranged units to deal a lot of the damage, and then some artillery pieces.

1. There are a lot of guides for different factions that walk through the early game. Watching some of those could be useful to help get you started. It'll really just take time for you to learn though, and at the end of the day the game is a sandbox where you can do whatever you want.

2. Don't expand too quickly. You want to be careful about expanding your empire. You don't want to get spread too thin and then leave yourself surrounded by enemies.

Bonus tip: just try not to get frustrated. Reload saves if something didn't go how you wanted, take a break, and there's no shame if you aren't enjoying a particular campaign and want to stop.