So, it's kinda easy, but also kinda difficult.

There's multiple sub-series.

If you want a more traditional game, start with Shin Megami Tensei V, the newest of the mainline series. You're generally safe just jumping in with the newest entry, they're not exactly the most heavily connected plots. If you dig it, you can always give the older titles a try!

They're turn-based RPGs(with a bit of a twist), but the main gimmick is that you can recruit enemies to come fight for you. Different personalities like different kinds of things, you have to work it out. You also can unlock ways to resummon and fuse demons to make more powerful demons, some of which are bosses(and deities. Like, Yaweh and Lucifer aren't unfamiliar faces at times in the series).

SMT: Persona is a sub-series that shares some likeness to the mainline(same demons, demons becoming a form of party member, resummoning and fusing), but takes place in the "modern" world. They also focus heavily on forming bonds with your party mates. While not unfamiliar to the SMT series, the Persona titles really bring it to the forefront.

Start with 3, 4, or 5. FES(PS2)/The upcoming complete edition for the remake, Golden, and Royal are the "complete" versions of each title, respectively. This is becoming the norm of the series, a base version is released and then an updated version with extra story content is later released.

If you get really curious, SMT 1 and 2: Innocent Sin/Eternal Punishment(It's two halves of the story for 2) aren't unplayable, by any means, but I would recommend having a guide on hand. They're much less polished than the later titles.

That's really the best I can think of for the "default" SMT titles that most would recommend, but other side titles like the Digital Devil Saga games have their own fun twists on the weird world(s) of the SMT series. Just remember that they're often some form of turn-based gameplay, and when dealing with earlier titles, notoriously difficult/grindy and occasionally obtuse. You're meant to take advantage of every weakness, exploit every option, but it will be returned onto you. Luck plays a huge factor in a bloodless victory or a total party wipe, though careful planning can give you an edge.

I'd say SMT4 on the 3DS is a good starting point if you wanted to dive in head-first to the mainline series. Otherwise maybe Persona 3 Reloaded for the Persona games.

Edit: A curveball good entry to the series would be Devil Survivor for the Nintendo DS. It's extremely easy to emulate with modern phones, physical copies are not as expensive as the 3DS games (but still expensive) and there is no region lock on DS so you have some lattitude to shop around if you want to use original hardware. In some ways that game does Persona better than the Persona games do, all while having a classic SMT mainline story of law vs chaos.

I think Persona 4: Golden is a good entry point into the franchise.

OK, so I actually know a fair bit about this series since I went through (a good chunk of) it semi-recently myself!

The mainline SMT games all take place in post-apocalyptic Japan, where your party is yourself, maybe a few other humans, and most importantly, demons that you recruit, level up, and combine together to make new, more powerful ones. Like someone else said, SMT is sort of like Pokemon, but instead of fighting with cute electric rats and furry bait, you fight alongside various mythological figures (...and furry bait). The SNES games are first-person, grid-based dungeon crawlers, but later games largely drop the grid-based aspect.

Anyway, I started out with Shin Megami Tensei 1 on the SNES. It was pretty darn enjoyable, though I used a walkthrough--if you play the SNES games, I strongly recommend doing this, because both games are basically one giant labyrinth with an overworld. A walkthrough is pretty much mandatory to navigate which demons are worth recruiting and merging together, and to find the various secrets and treasures scattered throughout the world. A nice thing about the first game is that the level scaling is well-paced; as long as you don't run away from battles and are smart about your recruitment and demon fusions, you should generally be able to keep up with the power level of your enemies.

As for SMT 2... well, it spikes the difficulty up much higher than the first game. to the point where I actually wound up giving up about 10-15 hours in, even with a walkthrough and using save states. I had reached a point where the enemies were outleveling my demons and killing them over and over, I couldn't easily afford to revive them, and I was having trouble recruiting new demons to merge with my existing party into more powerful ones--there were multiple instances where even when I used save states to explore the demon's entire recruitment dialogue tree, it either took my valuable items/money and ran away, or attacked me. Forced to choose between sitting and grinding for at least 5-10 hours, or moving on, I moved on.

SMT 3 on the PS2 is the first real "modern" shin megami tensei game, and it introduces the press turn mechanic that forms the core of the mainline SMT series from that point on. Press turns work by giving each side a number of actions they can take based on how many members are in the party--in other words, if you have 4 members active in the party, you have 4 actions. If you hit an enemy's elemental weakness, you're given bonus actions you can take (up to a max of 2x your base actions), and if you miss an enemy, or attack them with an element they nullify, reflect, or absorb, you lose turns. Crucially, this also applies to your opponents as well, making combat tense, tactical, and deep: your demon is the only one that uses ice magic, which the enemy is weak to, but your demon is weak to lightning and the enemy can use that element. Do you switch out this demon to cover your own weakness, or keep it in to better exploit the enemy's weakness? Remember, if the demon dies, you not only have to spend a turn summoning a replacement, but your baseline actions go from 4 to 3, so you're penalized twice.

Admittedly, I didn't play SMT 3 myself, because it has That One Fucking Spell called Beast Eye, which is something only opposing demons can use, and spends a single action to grant the AI two turns (or Dragon Eye, which grants four turns). This gives SMT 3 a reputation for being incredibly difficult, even by the standards of SMT, and frankly I had no appetite for that after having just given up on SMT 2 over difficulty. That said, everybody I speak to who has played SMT 3 says that it's one of the best RPGs on the PS2, however, so it's still highly recommended, and later games mercifully got rid of Beast/Dragon Eye.

SMT 4 is... odd. It starts out looking like a much more generic fantasy setting, but it most assuredly is not. It's good, but it also very clearly is straining against the limits of the system it's on. SMT 4 Apocalypse is also extremely good, and I would suggest playing SMT 4 just to play SMT 4 Apocalypse. I won't say too much about SMT 5 except to note that it's also good and I recommend it strongly.

There's also Persona. Where SMT is a post-apocalyptic dungeon crawler, Persona (at least from 3 onwards) focuses much more heavily on time management. You play as a Japanese high school student in Persona, so a lot of your activities are based around juggling a schedule: attending classes, going to after-school activities, working part-time jobs, spending time with your various party members to build relationships, and saving the world in between. Persona is also different in that instead of having mythological figures fight alongside you as distinct party members, they're instead Personas that act more like Stands from JoJo's Bizarre Adventure--they just give humans the ability to cast magic. Notably, the main character is typically the only one who can change their persona-- your companions all have their own persona, but they're stuck with the one they have, which conveniently gives them their own static elemental strengths/weaknesses and roles. The other big difference is that (up until Persona 5) the main dungeons were more roguelike, procedurally-generated designs, than the static designs of mainline SMT.

If you decide to play Persona, I'd start with Persona 3--either Reload (the recent remaster) or Persona 3 Portable (which has some extra content like that wasn't included with the remaster for some godforsaken reason). DO NOT start with Persona 5 like I did--to be blunt, it's way more polished than 3 or 4, and it'll be hard to go back and enjoy the previous games afterwards. You can also technically start with Persona 1 and 2, but they're waaay different than the later entries--they lack the time management/dating sim aspect entirely, and honestly there isn't a whole lot of reason to play them unless you wanna beat the shit out of Hitler for some reason.

Persona now very popular. I read digital devil saga good. Other game is soul hackers?

I think the newer games better to play first.