I've never created a theif type character and want to play some of the thieves guild, dark brotherhood quests. Can y'all give me some tips on armor types, weapons, spells, character race and such. Thank you, and be specific and give me personal opinions.

-Uses light armor
-Get all skills at sneak tree
-Choose Illusion or archery.With archery you kill enimies from above.With illusion you can watch them killing themselfs,and when you fail an assasination attemp.You can calm he victim.
-Uses enchanting,smithing and alchemy to create overpowerd equipiment
-Lockipick...you just need to the skill be high and have alot of lockipicks
-Speech...your choice
-Pickpocket it's good to sell stolen items at thieves guild to earn more money.

Bosmer or Khajiit. Light armor. Bows. Daggers. Illusion/Alteration Magic.

I use a bosmer with all of the above for that play style and it is quite fun. Especially with good illusion magic. Once you level up illusion quite a bit you get access to an invisibility spell which is perfect for that playstyle.

Sneak, Bows, Light Armor, Lock Picking, Pick Pocket, Alchemy for skills.
Bosmer or Khajiit for race, but honestly, go with what looks best for you.

Well ive heard some differing opinions on armor but personally I go this route: Sneak, archery or 1h (depends on my mood when i make the character, most go with 1h), usually one perk for illusion (maybe 2) for muffle spells, i dont bother with pickpocketing ( i never used it enough and potions can help with those more difficult pick pocketing assignments and lockpicking I find doesnt need a single perk to be effective at all). I choose light armor perks for the weightlessness and armor set bonus (though some will say heavy armor for the daedric weapons but I dont smith much myself and when I do its only to keep up with armor levels and upgrading, and who needs heavy when sneaking and not being seen? They should be dead before they turn around.) Again potions can help supplement any armor upgrade levels i cannot achieve without the related perks, and I don't need to invest anything into alchemy to have effective potions for these things and making poisons. And I find poisons extremely useful with a light armor thief/assassin.

In my let's play I level up
Stealth
Illusion
Archery
One handed
If you get invisibility being a thief is easy

I take it Bosmer and Khajjit are best suited?

What everyone else said. Stealth is a must ofc.

any race can be a thief just bosmer and khajiits have better starting stats for a thief

You've already got suggestions for all the skills you might need...
So my advice is becareful not to level anything too quickly and definitely don't feel the need to grind out a skill.

Theives make use of a lot of non-combat skills and as these level up you'll find your enemies getting tougher. So if you notice your non-combat skills raising your level, make sure to work on your combat skills more or else you'll find combat alot harder where stealth just isn't a viable option.
Also do your research on the skills and perks. Since we make use of alot of skill areas don't waste those perks in something like lockpicking, which a master lock can be done even without perks.

While you might choose a certain style, bow, daggers, supplemental magic to aid your theif skills. Balancing your Health, Magic and Stamina early will help.
However Magic and Stamina have limits of usefulness to be raised to a certain point, afterall you can easily cope if your magic and stamina run out, your health not so much.

What everyone else said except, use one-handers instead of daggers.

A: Daggers are one handed
B: Daggers are the only one handed weapons that make sense for a stealth character due to the massive damage boosts from the Sneak perks.

My theif/assassin is Woodelf using two daggers and a bow, the only magic I use is resoration now and then to heal. Main perk focuses are: sneak, archery, 1 handed/duel wield, lock picking, and light armor. I do have a few points into pickpocket should I need it but I don't use it often.

I strongly disagree, they still do low damage compared to other one-handers and the 6x damage to one-handed is enough to instantly kill most mobs.

There is a sneak perk that allows daggers to do 15x damage
Plus it is really cool to sneak up behind someone to kill them and have the animation where your character covers their mouth with one hand and open their throat with the dagger and have the enemies body just fall at your feet ... not sure if you can do that with swords

I know.. not sure what your point is.

Daggers make no noise when you use them. One-handers and in particular two-handers makes a lot of noise, which will attact nearby enemies.

Use daggers for backstabbing, one-handers for mano-a-mano.

Acually, the highest dps combo is dagger LH for the speed and sword RH for damage.

That's a descent argument for daggers.

But at low levels playing as a dagger wielder definitly isn't worth it.

Orc
Heavy armour
Two handed
Smithing
Archery
Block

Make sure to be a Werewolf.
As long as you steal you're technically a thief, no?

The Pentilus occulus armour/boots with Ancient Shrouded Gloves/Cowl looks just.......great!

With the sneak perk + the DB gloves you can do 30x damage with a sneak attack.

You pretty much one hit kill every enemy in the game... daggers are definitely the way to go for a sneak char.

Only if you want to be a truly evil character.

And still you get 12x damage with other one-handers which is also enough to kill most mobs.