Hello!

I'm trying to build a metallic dragonborn hexblade warlock using glaive. And I'm also planning to use PAM later.

I choose my race because 2d10 damage breath weapon and metallic breath weapon can give warlock AOE abilities without spending any spell slot, and dragonborn looks AWESOME!

But I'm not really sure about my character. Even though I'm not doing min-maxing.

I will get Gift of the chromatic dragon feat so I can get some free resistance and a little damage boost which doesn't require concentrait. It also fit perfect to dragonborn.

Eldritch Invocation will be Thirsting Blade, Devil's Sight(Dragonborn doesn't have Darkvision),  Eldritch Mind(or Eldritch Smite and Improved Pact Weapon?)

And my spell list will be

Cantrips : EB, GFB, Minor Illusion

1st : Hex, Armor of Agathys

2nd : not sure

3rd : May be Fly?

But I don't know what other spells should I take.

I know Darkness and Devil's sight combo is strong... but I also know it makes GM and other players boring.

What spells should I take? And Flying hexblade is good choice? What about Eldritch Invocation?

I'm sorry English is not my first language so my question might be awkwared.

It can work fine without Darkness. Darkness + Devil Sight is very powerful, of course, and it is the most optimal. But you can play without it and be perfectly functional.
The breath weapon + Armor of agathys will make you very powerful against crowds. lvl 2 I would put misty step. By itself it's a very useful spell, but it comes in handy with armor of aghatys. Why? Well, one of the problems that armor of agathys has is against entrenched enemies, who take it from you with ranged attacks and don't suffer the consequences. With misty step you can get into the middle of a horde of goblins hiding behind a barricade, and they die alone.

Thanks ,I didn't think about that spell! Because you know, many people use free Misty Step by feats or race feature. Maybe I felt it's unfair to use it with spell slot.

Now I should look for some teleport spells. Like Misty step and Thunder step, I need to think wich is better(MS works on bonus action but I can't upcast, TS works on action but range is 90 feet and it also deals damage).

I'm curious why you might have thought getting Misty Step through Spell Selection instead of Feats or Racial Features was unfair.  Getting it through Spell Selection (or copying, if Wizard) is the expected method for obtaining Misty Step from the Players' Handbook. Feats and Racial Abilities that give Misty Step came in later sourcebooks.

Obtaining it through a Feat or a Racial Feature often provides additional spells or abilities, or an extra chance to cast the spell without using Spell Slots, and often lets the character cast the spell using Spell Slots, as well .

I started playing D&D this year. So to me, almost every source books were natural thing.

Many players I met often use Fey Touched or Eladrin race and nobody picked Misty Step normally.

So I think I misunderstood that "Misty Step is for half feat or racer abilities".

GFB isn't going to be very useful when you have a breath weapon you can throw out for aoe.

I'd take mage hand instead, and use it to carry a light source for you when needed.  Then you can drop devil sight since you aren't using darkness, and take another invocation.  

You'd still be able to see, and you get the utility of mage hand, and another invocation

Quote from volcanic_rock >>

I started playing D&D this year. So to me, almost every source books were natural thing.

Many players I met often use Fey Touched or Eladrin race and nobody picked Misty Step normally.

So I think I misunderstood that "Misty Step is for half feat or racer abilities".

The best race for that is the Shadar-kai. First thematically, since the raven queen is the paradigmatic hexblade patron. But also because her racial "misty step" gives you resistance to all damage for 1 turn. That makes the combo with armor of agathys + Shadow of moil very powerful. If they hit you, they do half damage and they get punished with a lot of damage back.

Utility spells are great. As far as invocations go, obviously thirsting blade and eldritch mind are must haves. The final one devil sight is fine for what you are going for, though others are good to, like Eyes of the Rune keeper, I always like Eldritch sight for at will detect magic, at will spells in general are good. 

Darkness is still good to have if you have devil sight.

Other spells that are great are, invisibility, hold person, fly, tongues, blink is a no concentration defensive spell on the hexblade list. Suggestion is always a good spell to. Hypnotic pattern or fear is good to have. I personally love charm person.

Combat wise you are mostly covered with just pact of the blade PAM and your dragon features. So it is just a matter of what else.