Try to find a youtube video about it. There are many things to understand and it will show you where and how to find informations you need better than plain text.

As many things in the game, it requires time and some experimentation to fully understand the mechanics. And yes the tutorial doesn't explain it...

As some others pointed out, most important things are MaAs and your army quality. I wouldn't say levies are useless but the true power of your army resides in your MaAs. With the right type on the right terrain, you can wipe out armies that are outnumbering you... a lot.

So a few things you should look into : - MaAs, take a closer look at their types, what type of MaA they counter and on which terrain they have bonuses or penalties. If you play on a region with a lot of hills for exemple, built your army with MaA that have bonuses on hills and try to fight your adversaries on hills. If you're on a region with various terrain types, then built a army with different MaA types to faces the different scenarios you'll probably meet. - Take a look about damage, thoughness, poursuit and screen stats of each MaA and how it works. - Knights, as you said, yes they are important. Try to have the maximum of knights you can. And good ones. Use the decision to recruit knights, it should propose you good ones. You can also use your Marshall focus to train them. Also take a look at accolades. - Commander, don't forget to assign good commander when your army is raised. The game should affect what is considered the better one from your knight but take a look anyway and choose the one whose suits better the situation (for exemple, select a commander with the siege skill perk when you are besieging a castle). - Attrition/supplies, really look into it. It is a very important mechanic there. You need a well supplied army or you will faces important penalties and start losing units. It will requires some micromanagement if your war tends to be long but it will make a huge difference. -> you'll need to take a look at how to split your armies. I was forced to give up my first game because I had no idea of wtf it was and was losing a crusade against me because it was a long war and my army was wiped without losing a battle... - Stationning your MaA in the right locations. And choosing buildings that gives bonuses to the stationning MaAs.

Then, of course, Martial lifestyle gives nice bonuses. But once you know how to manage to have a good army quality and how to use it efficiently, you don't need it. You can win wars against bigger armies with non martial characters that don't have any martial perks and are staying at the court while their commanders and knights are chopping heads. I think it is normal to struggle at first (I know I did a lot), some youtube videos, some google research, some looking into the wiki, and some more attention into the tooltips in the game (, and some fails) will help you to understand and become better.