Hello everyone.

I am DM'ing for a group of first time D&D'ers, me myself is also a rookie regarding the pen and paper RPG.
I have a question concerning magic items for casters.
Fighters / Clerics and melee characters make good use of +1 weapons and +1 armor maybe with a small bonus effect.. But what items / armor / weapons are nice for casters to get? Because we have a Warlock in the party and i find it hard to find some useful items to give him, so that it is not only the melee characters getting the "YAY" effect.

I hope you can help :) 

A warlock would love a rod of the pact keeper, and any of the squishier mages will benefit from a cloak of protection

The most useful item my wizard received in our last game was an Amulet of Health. Kept me from almost dying every session!

Any wand, especially Wand of the Pact Keeper or Wand of the War Mage. Staff of the Adder or Staff of the Python are also great for Warlocks. If he took the Book of Ancient Secrets invocation, he will love spell scrolls with ritual spells on them too. 

Wand of the war mage. Ruby of the war mage for magic swordsman type characters.

Given the limited number of spell slots available to a Warlock, acquiring a Ring of Spell Storing or several can be a great asset.

Cloak of Protection
Ring of Protection
(Note: the cloak and ring stack which is why one is rare and one is uncommon)
Cloak of Displacement
Rod of the Pact Keeper
Any wand (If a Hexblade or Bladesinger a Wand of Paralysis is brilliant: multiple attempts to Hold Person, no spell slot cost, reasonable DC, and if successful that's auto-crit on all attacks made within 5 ft: combine with Booming Blade or Green-Flame Blade or even better, Eldritch Smite).
Any Staff
Ring of Spell Storing
Pearl of Power

Just the ones off the top of my head. 

Ring of Spell Storing is kind of weak for warlocks unless you have another caster in the party who can cast lower level spells into it for you. It holds 5 levels worth of spells, so after a warlock hits 5th level they can only store 1 spell in it themselves because all of their spells, even 1st level spells, are upcast to 3rd level spells.  I played a warlock who found one once and he had another caster in his party stock it up with spells for him when he had the chance because he could only put one spell into it. 

Quote from Tim >>

Ring of Spell Storing is kind of weak for warlocks unless you have another caster in the party who can cast lower level spells into it for you. It holds 5 levels worth of spells, so after a warlock hits 5th level they can only store 1 spell in it themselves because all of their spells, even 1st level spells, are upcast to 3rd level spells.  I played a warlock who found one once and he had another caster in his party stock it up with spells for him when he had the chance because he could only put one spell into it. 

 
It's still more spells than they normally can cast, a big thing for Warlocks. So, it's still worth it.

Quote from Tim >>

Ring of Spell Storing is kind of weak for warlocks unless you have another caster in the party who can cast lower level spells into it for you. It holds 5 levels worth of spells, so after a warlock hits 5th level they can only store 1 spell in it themselves because all of their spells, even 1st level spells, are upcast to 3rd level spells.  I played a warlock who found one once and he had another caster in his party stock it up with spells for him when he had the chance because he could only put one spell into it. 

A Warlock's spell slots will level to 5th level slots at Warlock 9.  At that point, a Warlock has.....  2 spell slots.  Warlocks do not get their 3rd spell slot until Warlock 11.

That's also why I said "or several" when suggesting Ring of Spell Storing.  The idea is that as a Warlock, you'd want to get as many of them as you can afford Attunement slots for (so up to 3 Rings).  That would take a Warlock with 5th level spell slots from 2 to 5.  That's huge for just Armor of Agathys and Witch Bolt alone -- both do crazy amounts of damage, and Armor of Agathys is a substantial survivability boost.  Refreshing spell slots on Short Rest from being a Warlock also allows you to refill them much more quickly than other classes.

Thank you for all your nice suggestions!! It is so nice with such a great communty for this game!!

I think for the lower levels he will get the Ring of Spell storing and Cloak of protection

What level should i award the Rod of The Pact Keeper +1? I think around level 5! Or should it be +2 for a level 5 character?

What are your thoughts?

Again thank you for the amazing feedback! :) 

If you look at the rarity, other than potions and scrolls, each character “should” have one magic item of each rarity when they hit 20th level, starting with an uncommon item and getting them in order. The suggested rate is to get one item at each Tier of play. I’m playing a 7th level Moon Druid right now and he has a Ring of Water Walking and Bracers of Defense, which is just about right.