I've been playing 40k since '98, and have gotten dozens of people into the hobby, there are three things I recommend people take into account when picking their first army, in order of importance.

#1: What the minis look like. A huge portion of the hobby for most people is building and painting, so you really want to go with minis you like the look of. You seem to have narrowed your choices down on this.

#2: Lore. The game changes every few years, but lore is pretty consistent for the most part, read up about your favorite armies and pick the one that speaks to you the most. You may love the look of one army, but if the lore doesn't speak to you that could cause you to lose interest.

#3: Play style: To me this is the least important factor to choosing a first army for a couple of reasons. You don't really know what play style you enjoy, you may think you like static gunlines, but actually enjoy zipping around more. Most armies actually have units that can fit multiple play styles, and your first army will likely be in the 500-1000 point range, which means a couple troop options, an hq, and a special unit or two, and once you play a few games, you can decide to focus on more heavy support, or fast attack, or elite units as you get the feel for what you like, beyond that, the meta changes, one edition may be more heavy weapons focused, one may be more troop focused, a few years back mechanized was very powerful, the foot slogging infantry was the way to go. If you pick an army based on aesthetics and lore, you can (and likely will) expand that army over the years to be able to field a force that has a number of play styles.

The one thing I recommend every new player avoid is meta chasing. Don't pick an army because it's powerful or wins tournaments, because in 3-6 months that will have changed, and you may be left with an army that doesn't interest you.

I think the best way to pick your first army is to try and find one who's troop choices you'll enjoy painting the most. If the troops are really interesting to you and you'll enjoy painting them, that means you're more likely to finish painting them and be encouraged to play games. I guess this doesn't apply if you want to play with unpainted minis though.

I picked the one I liked the lore of most

I liked the look of the eldar vehicles so I went with that. To be fair I was like 13

It’s so dumb. I wanted to start the Tyranids cuz they looked cool, but my mother said they looked too evil (good thing I didn’t show her Chaos or DE) so I went with Tau instead. Later picked up Necrons because they gave off major “cosmic horror” vibes.

While I appreciate the newcrons because now they have actual characters/stories (who doesn’t love Trazyn) and new models, there was something really cool about the more mysterious 3rd edition Crons that has sadly been lost. Also rip Pariahs.

It's not really the question. How the hell do i stop buying armies? You might not be there yet.

But the first was the colors green and white of the bel tain eldar

Helsreach. Crusading knights in space trapped in a sci-fi siege of a walled city? Yes please. See the video adaptation of the story below that was later made by a fan of the story (and eventually taken down by games workshop to be moved to their Warhammer+ service but has been re-uploaded by someone else, but that's a long story in itself).

https://m.youtube.com/watch?v=s2WGE1L6WKs

I loved that the orks weren't as so self-serious, and are just so ridiculous so I went with them because they're just pure fun. Later as I got more into the lore I started an Aeldari army because I liked theirs a lot.

I picked my first two -- Necrons and Chaos (Thousand Sons) -- both because I liked the lore behind them (at the time that I was playing; current Necrons have completely lost the thread). I've always been terrible at the game but the lore keeps dragging me back in.

Have you seen the Grey Knights models? Have you seen them?! I rest my case 😊

For me it was a mix of looks and lore, and I chose thousand sons.

If I was going to get another, it would Tau though, because of the lore.

I was always intrigued when I saw SM models in hobby stores but as a teenager the price just didn't make sense... and I had to paint them?? How was it possible??

Then much later, as an adult with a lot of spare time during COVID, I picked the faction which had a color scheme I liked, checked if I liked the lore, and decided to pick up another hobby in mini painting and paint a few units to make a small band. That small band became a plastic crack addiction with a desire to have a whole army regardless of playability.

My father brought me into a hobby shop that had a bunch of second edition 40k models on sale. We had recently played the Space Hulk game, so I was familiar with elements of the 40k universe.

I saw the Chaos Space Marine range and was 100% in. I came home with 2 boxes of the cookie-cutter plastic marines, a squad of metal havocs, and the metal champion with the four-fingered power fist. The second edition Chaos codex cover was straight fire as well.

Good times!

I was hungry when browsing thru codexes at LGS. The extra galactic locust swarm seemed to be hungry too, so I went with Tyranids.

I haven't had the expendable funds to actually start yet, but I've decided on 'nids. I absolutely loved Starcraft and the zerg were my favorite race. And the 'nids are very similar in aesthetics and lore.