So my general concept is that a powerful Archfey queen/princess REALLY wants her own "Prince Charming" like she reads in stories. So she blesses a small warrior that get's lost in the Feywild, trying to turn him into her ideal knight. In general, I'm aiming for elements of that traditional "Fairy tale" vibe, as well as some of the whimsy of the feywild, while also making him somewhat viable. 

I should also note that it's gonna be a level 11 campaign, and I'm fine with taking multiclass dips if need be.

Assume you want a 2024 version

Taking a dip to either fighter or ranger gives you that access to (shining) armor.  Taking a Barbarian can give you a decent AC without shining armor, but no spells if raging.

For Lvl 11, determine if you want a warlock level 10 or level 9.  That gives you the dip.   

Not sure why you want a warlock to be prince charming, not sure what it is to make it a viable choice to be choice for warlock?  That may be a bigger reason to determine you warlock level.

Is there some sort of Eldritch Invocation you are looking at?

I like these Ideas a lot! As for my reasoning, I wanted to experiment with a Melee warlock, but do something really interesting, tying into my love of subverting tropes. I liked the idea of some random traveler that's lowkey FORCED to be some Archfey's knight in shining armour.

Take at least 7 levels of warlock, and 3 of fighter.

The 11 level is your choice as either one will give you ASI.

3 levels of fighter gets you the Champion subclass.

Level 7 & 8 give you 6 Eldritch Invocations.  Level 9 gives you 7.

I recommend you to have 

Pact of blade is a must.  your CHA bonus is used instead of strength, so you can pour all of your ability increases into CHA. and have a good melee weapon.

Agonizing blast - If you take the cantrip  Eldritch blast and this invocation, it can act as a Bow weapon.  That means you do not need a high dex in this build.  You have a nice ranged weapon that scales on your total level, not your Warlock level.  so a 11 level character gets 3 attacks.  It uses your CHA bonus for damage.

Thirsting blade

The others are determined by more along your style of play.

Eldritch Smite uses a spell slot and at level 8 you have but 2.  So if you cast any spell, that leaves you with one slot.  (if you have no intention of using spells, get this otherwise give it a low priority.

Fiendish Vigor gives you some temp HP (=12) and is a decent one, but not needed.

Pact of the Tome can give you more spells, but you may not need them being a warrior.

Lifedrinker is good, but you need level 9 warlock so lower fighter levels.

If you want spells, Eldritch mind gives you an advantage for concentration spells.  a fighter gives you bonus to con saves. 

Lessons of the first one can get you some origin feats.   That can give savage attack/alert/tough so a good one to get if you still need to take an invocation or two.

If you want to have familiars, which also lends it self to more spellcasting, Pact of the chain & gaze of two minds are good, but again, not the best for melee.

If you go ranger you need a high wisdom for those spells, so fighter gives you just a need for CHA ability for spells.

If you go melee warlock/fighter.  You do need CHA but not need strength.   Your dex is needed for initiative and AC.  Heavy armor does not use dex, so dex is no longer as important.  Con is needed for HP.  Having a d8 vice d10 for HP can be an 11 point difference.  However Fiendish Vigor gives you 12 points, so that is now a wash.  

Thanks, this is gonna help a lot! Just wondering, how do you think it could theoretically pair up with Paladin instead of fighter? I know warlock and paladin share a spell casting mod, and maybe paladin could fit that "Heroic" vibe. I'm still deciding between the two, but I wanna know what you think would be better.

Not sure on paladin.  But the same basic framework is there.  

Looked at paladin.  Other then the name, not sure you really get much.  A 1 level ftr gets what it takes to get 2 levels of Paladin.  To make a difference you probably need 3-4 levels of paladin.     That goes to how much warlock do you want?

I have been thinking about this exact character for a few days.  Here's what I was thinking:

Likely human, child (size small).  Literally a kid playing at a Knight. Ends up in the feywild, serves their Lord or Lady.  Play up the most fairy tale knightly ideals you can.

Between stats and background get a 14 dex, 16 con, and 17 cha.  Likely dump str and int.  Take Purple Dragon Rook as your human feat for knightly flavor. 

First 4 levels are Warlock 1 Bard 3 (valor), your choice of which to start as, though likely warlock for blade pact unless you're starting above level 1.  Need valor bard to get medium armor and shield asap, through it should still look like a cobbled together kids armor.  Booming blade to boost your weapon, which is a kids wooden sword (club).

From there you want to get to warlock 3 for archfey and bard 6 for extra attack.  Agonizing blast on booming blade to make that attack hurt, as well as any opportunity attacks by taking War Caster as your first feat.  You can potentially look into getting shillelagh to boost your base attack damage too, either via magic initiate or Pact of the Tome (definitely the latter if your DM might allow you to take agonizing blast for shillelagh as well, dubious though that might be).

From that framework you can go where your heart desires. 

I know this is a Warlock idea, and referring back to 2014 rules,  but the Fey Wanderer Ranger seems like a good candidate for a "Fey Knight in Shining Armour". Assuming you strut about in medium armour you could pretty much focus on Wisdom and Charisma (using the aboive mentioned Shillelagh cantrip for melee, and maybe produce flame or magic stone for ranged, both obtained through either magic initiate feat or druidic warrior fighting style) so a multiclass with fey warlock might be possible for some extra fey-ness.