Warhammer 40K has a massive tome of rules to wade through and understand.
From there they are every changing due to updates from FAQ’s and Chapter Approved.
Pile on all the different armies, units, special models, and limited release stuff…
…and you have quite a pile of stuff to understand.
Where to start, and how to start winning most games.
A pre-assumption here is that you already understand the core rules of the game, not from a have-them-all-memorized point of view, but rather as you understand the flow of the game and the win conditions for the missions.
It’s ok if you have to pause and look something up in the rules.
Where you start is with how your own armies plays.
Eldar, Tyranids, Necrons, whatever you are playing- what is the plan for the units you have.
Maybe it’s a small starting army, or a mish-mash of stuff that you got in a trade.
A specially constructed list, either way, what is the plan for using it on the table?
A number of your first games are going to be learning and remembering how your army works on the table. As you become more familiar with it, and it start to play without you having to think about it, you will begin to see opportunities and mistakes made by other players as you battle.
Be ready to seize them.
But this is only half the win formula for 40K.
The second half is understanding the meta of the game- what is popular in the moment and what is currently being played, be it at the local gaming club, or the tournament scene.
Wherever you hang and game, you are going to come up against them meta.
Games Workshop is constantly releasing new units, books, and changing the rules. Armies rise to the top of the tiers as other fall. A single new unit that is overpowered and gets spammed out, can shift the movement of the meta overnight.
Games Workshop is many things, but balanced games is not one of them.
Where you are playing, what is the BULK of the armies people are playing.
But if you are in a place where 5 out of the 10 players are playing Tyranids, that is where you start.
You start learning about the other 40K armies and units based on what you are most likely to face.
30 out of 50 Grey Knight armies in the room, worry about Chaos Space Marines for another time.
Sure, you might come up against that one guy playing space elf space clowns, but always play the odds in 40K, especially the meta.
Learn to beat the most popular armies in your location FIRST.
Follow up with the rest.
THAT is how you increase your win rate in 40K fast.