Playing a 2nd level warlock about to reach 3rd level, and so far all i do during combat is to either cast eldrich blast or to first cast hex as a bonus action, then followed by eldrich blast on whatever target i have cast hex on. So when i reach 3rd level and pick pact of the tombe, i will probably pick at least one damage dealing cantrip as one of my three cantrip choices and i'd like some advice on what cantrip to pick, given that it ought to in some way compliment eldrich blast a bit

(some random bits of context: i do have repelling blast but i don't have agonizing blast, so EB will probably not be that far beyond any other cantrip i pick in terms of raw damage, but also something like ray of frost might be redundant, my dexterity score is 10 so booming blade/ green-flame-blade is probably out unless i also pick shillelagh, playing decent into avernus to presumably things we encounter might start resisting acid/cold/fire/poison pretty soon and also my fellow party members tend to be quite reckless / stupid so maybe anything that would help me make my party members not die could be useful?)

Toll the Dead (Xanathar's Guide to Everything) is the easiest to recommend as it does solid damage compared to Eldritch Blast, has a good damage type and uses a Wisdom save so does the damage pretty reliably if you have a decent save DC.

Does your party have anyone else with save-based spells or abilities? If so then Mind Sliver (Tasha's Cauldron of Everything) could be a good option, as while the damage isn't amazing, it's one of the few cantrips targeting Intelligence saves and not especially well resisted damage, but crucially that -d4 to a saving throw can help to ensure success of something clutch like Hold Person or similar abilities.

If you don't mind taking another attack cantrip then I'm a fan of Chill Touch as it prevents healing and imposes disadvantage (vs. you) on the undead. However conventional wisdom is that if you already have a good attack cantrip then you probably want to pair it with a save based one, as that may work better against high AC enemies.

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Toll the Dead (Xanathar's Guide to Everything) is the easiest to recommend as it does solid damage compared to Eldritch Blast, has a good damage type and uses a Wisdom save so does the damage pretty reliably if you have a decent save DC.

Does your party have anyone else with save-based spells or abilities? If so then Mind Sliver (Tasha's Cauldron of Everything) could be a good option, as while the damage isn't amazing, it's one of the few cantrips targeting Intelligence saves and not especially well resisted damage, but crucially that -d4 to a saving throw can help to ensure success of something clutch like Hold Person or similar abilities.

If you don't mind taking another attack cantrip then I'm a fan of Chill Touch as it prevents healing and imposes disadvantage (vs. you) on the undead. However conventional wisdom is that if you already have a good attack cantrip then you probably want to pair it with a save based one, as that may work better against high AC enemies.

 well we have a bard with thunderclap & earth tremor, a paladin who will probably never use his spell slots for anything other than cure wounds or a mundane smite, a rogue belonging to a player that has not been with us for the last 2 sessions and that i have more or less controlled so far, a wild card in the form of a new player whose class and tactics are as-of-yet unknown and as for myself i plan on taking fireball but don't exactly have any save spells yet, so the synnergy is perhaps absent but certainly there to some degree (and if nothing else, mind sliver always goes well with more mind sliver). That said it would probably go really well alongside summon undead spirit (putrid or ghostly), and since my character's backstory involves studying under a fiend becuase he felt that the elvish tutors in his magical school were too cowardly to teach him the truly grim stuff, that is certainly a spell i am interested in.

I already picked protection from evil and good so if i want to give an undead creature disadvantage on attack rolls against me i would simply do that, but since i already got hex and he is at the very least necromancy-positive it is a possibillity.

Might be dull with toll of the dead as another cantrip that does nothing but damage, but at the very least having to specifically target creatures that are already harmed and not having to use it on the target of hex might be enough? I don't know

Currently playing a Tiefling Celestial Patron pact of the Tome, with a level of Knowledge Cleric.  As such I have access to an insane number of Cantrips.  There are really five that I consider interesting to add onto an EB Warlock.

1. Shocking Grasp.  So this is a melee range spell, but uses your caster stat to hit.  Normally a Tome-lock does not want to be in melee, and that is why you may want this spell.  If the enemy has metal armor you have advantage on the attack making it easier to land.  Most importantly  if you do land the attack the enemy "can't take reactions until the start of its next turn."  This allows you to move away and reposition yourself.  There are a decent number of lightning resistant/immune monsters, about the same as cold.

2. Word of Radiance.  This is your I'm surrounded cantrip.  You likely won't use it much, but when you do it can be very impactful.  Radiant damage is rarely resisted, and most critically the spell allows you to exclude allies.  Combined with misty step you can get away as well.

3. Sacred Flame.  Another radiant spell.  The critical part here is that it ignores cover on the saving throw. If you don't have spell sniper this can be really fun when an enemy decides to crouch behind something to be protected from you spells.  Its a save, but the 1d8 damage die is comparable.

4. Chill Touch.  I killed a troll with this spell.  The rider that the target can't recover hit points is useful quite often, and can ruin a DM's day when they want to keep an enemy alive.  You can get this from the Warlock list so I do not recommend stealing it with Tome.

5. Mind Sliver.  The ability to do damage while debuffing a save is powerful, but you are mostly here for debuffing a save.  Its on the Warlock list so I do not recommend stealing with Tome.

My suggestion here is that perhaps the answer lines in 'tactical' cantrips. Nothing is gonna be on par with an Agonising Eldritch Blast in terms of damage, but cantrips like Create Bonfire is useful in corridors or to pair with your Repelling Blast (though fire resistance will mess with this).

Word of Radiance is handy if you find yourself surrounded by enemies for whatever reason - judging from your Dex score, you don't want to be caught in that position but with the number of options you can get, it's worth a place.

And then, yeah, pump out of combat cantrips that you couldn't otherwise get, like Guidance (since your party seems it might need some extra skills support without a bard and an inconsistent rogue). 

Primal Savagery is a D10 melee attack spell that would give you something to do in melee. Eldritch Blast with Agonizing Blast is the king of damage cantrips, and if you need anything on top of that go with with Toll the Dead, but as one of your Warlock spells, not one of your Tome spells. Pact of the Tome allows you to get three cantrips from other classes lists, so never grab anything you can get from Warlock.

Guidance is pretty much a must have, as it's a free 1d4 to any ability check. You should always have this cast on yourself, except when you're holding concentration on something else, or giving Guidance to another character.

The third is basically up to you. Since you have ranged damage and melee damage solved I'd suggest some sort of utility cantrip. Mending isn't on the Warlock list for some reason.

Toll the Dead as damaging cantrip is always a good option, Vicious Mockery is not to be overlooked as it gives disadvantages to the next attack and can help to keep your party members alive. Spare the dying for those same reckless players can always come in handy.

Adding to Nechaef's comments, Vicious Mockery is great fun to use if you like to say your chosen insults. Some prefer old memes, others like to do Shakespearian attacks. Whatever floats your boat.

Personally, I like, "Your fly is undone," and hitting them with a save or suck when they are distracted. 

I think you kind of need to go into playing a warlock more like a martial character than a spell caster. Your action economy looks a lot more like the Arcane Archer's than a divination wizard's. With that being said, having a save based cantrip can give you something else to do, but expecting variety out of a warlock seems like barking up the wrong tree.

Guidance, minor illusion, prestidigitation, mage hand, message.

If you're just looking for damage cantrip options, Vicious Mockery and Shocking Grasp would definitely be high on my list. Shocking Grasp gives you a nice melee option that lets you back away from the target, and Vicious Mockery is nice because psychic isn't resisted all that often and nerfing your target's next attack can be quite helpful. I've come to love Mind Sliver with my current Warlock, it's always fun hearing the DM groan when I tell them their monster has to make an Int save :) As said above, Mind Sliver is already on the Warlock list though, so unless there are other Warlock cantrips you really want, it's probably not worth grabbing as one of your Tome cantrips. I also tend to take at least one non-damage cantrip when I go Tome, something like Spare The Dying, Thaumaturgy, Mending, etc. Sometimes it's for utility, sometimes it's just for fun RP potential. 

I know it could be considered meta-gaming, but in the back of my head when playing a Tomelock is always "I need something I can hit a Helmed Horror with!" which is why I rarely go with something necrotic-based for my backup damage type :) 

I absolutely agree with what most people have said - Toll the Dead for great damage (especially when an enemy already lost some health points which is the case like always) or Mind Sliver for not-so-great damage but the addition of its side effect (-d4 from enemy saving throw) and INT save. Chill Touch is also good.