Well there's a lot of summoner builds, that play fairly differently, from very passive "run around and wait for shit to die" to "casts a few spells to buff/debuff your pets" and finally stuff that is constantly casting, summons or others.
The main summoner variations would be, and the list is non exhaustive:
- Spectres, fairly strong, pretty passive
- Skeletons, worse clear speed but better boss killing speed than spectres, comes with a vaal skill(shits out 40skeletons all over the screen), pretty active since you have to resummon a lot, can do Skeleton Mage variation but requires more ressources and isn't worth it
- Golems, very expensive to build so not recommended for a first build, fairly passive but very strong
- Summon Raging Spirits(SRS), easy to play but a bit weaker than some other builds, pretty active since you constantly have to summon
- Animate Weapons, clunky as shit, not very strong, not worth doing currently(Chains of Command version might be a bit less shit, but still meh imo, the clunkiness is really bad)
- Herald of Agony, plays very different than traditional summoner since you need to be procing a lot of poisons to maintain it, can be played as different classes as a result(Ranger for example), very strong
- Dominating Blow+Herald of Purity, melee summoner, not that strong due to the melee nature and low boss proc rate but a fun change of pace
- Soulwrest Phantasms, fairly potent, has one of the better clearspeed for summoners, but as the name implies, it requires Soulwrest(a unique staff) to even work, so you need to lvl as something else and then invest in the staff to get started
- Zombies, hipster-ish build, require fairly specific gear to make work decently and even then still not really worth, AI is fucking retarded
Note that many summoner builds will actually use several summons. It's common for most builds to run either zombies or spectres on top of their main builds(zombies for meat shielding, spectres for either additional clear speed or for buffs), or both, or even skeletons too(for single target damage). Also summoners have a ton of variations in terms of how they'll build each, from the basic life, hybrid or ES, to even ascendancies(mostly Necromancer obviously, but Occultist can work and for the Herald based ones, a lot of other classes work).
Best is to just start googling summoner x guide and just see what you find, especially youtube guides to see how they play, and figure out what you want.
I'd say the basic starter summoner would be a life spectre necro variation. Either with skeletons, or just raw spectres(that's still probably involve zombies and animate guardian for meatshield/buffs). That's your vanilla summoner experience and clears pretty much all content without gear or requiring indepth knowledge of mechanics. Herald of Agony build could be good too if you care more about power since those builds are very strong currently, but they might feel the least "summoner-ish" of the builds, with many just playing with a Bow and having just the one Agony pet.