Minor spoilers alert although I don't think anyone starts playing on dark for the first time. Here is everything you need to know (EDIT: added a few edits and tips):
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STORY decisions: Go with Iorveth. His route is easier and you have 1 to 4 allies in every major fight beside you - meaning you can backstab enemies without a problem. There are plenty of opportunities to avoid fights in the story if you choose the peaceful option or go around: persuasion options, sneaking through the camp / caves, sparing Letho etc. Avoid these fights! Always save. You probably won't die much but you'll be glad you saved. EDIT: longer but safer if you do (almost) every side mission (you will need 8 grapeshot, 7 harpy traps and 80 harpy feathers if you don't want to backtrack). The one I suggest to skip is the cursed medallion in Flotsam (just sell it to the witch) and don't try to give a false receipt to the whispering man in the harbor (his henchmen are too powerful early in game).
Must have optional KNOWLEDGE: you'll get these free if you do the following. First is the Assassin ability (+25% dmg for backstab) for finding a dead Assassin's Creed assassin. After you reach the castle walls in the prologue Foltest tells you to help his men get to a tower and you need to climb down to a ballista. Before you climb back go as far as you can behind the ballista and you will find the body in a haystack.
Second is +10% experience for slaying 10 training pole. Normally you can't get this early in the game but when you enter Flotsam for the first time and go straight you'll find a door leading to the forest (this will become unlocked after you saved your friends from execution). There are two RESPAWNING pole in here. Destroy them, go back to the town, then back to the poles. Repeat. (EDIT: there are 5 training poles in the prologue where the first arm wrestling and dice poker location is, if you don't slay these you won't get the ability using the respawn trick!!)
Third is strong back. You can carry more loot (+50). You get this simply by persuading the La Valete prince to surrender when you get on the top of the tower (and you'll need to carry him to the secret escape route in the dungeon).
TRADING: all you need is 10 cat potion (you can make them easily early) and one Mongoose potion for the Kraken. The only armor / weapons you need are the dark exclusive outfits (Blasphemer, Oathbreaker, Kinslayer). Kinslayer is not even needed, the hardest enemy, the dragon can be defeated (how to defeat it is in the combat section) with the Oathbreaker. The other enemies don't become stronger so except a few boss battles you'll fight the same enemies. You can find the diagrams for the outfits at the local blacksmiths. The most important is that you'll need to write down the crafting ingredient for the outfits and sell everything else!!! You don't need potions (except the two above), oils, bombs, daggers etc!! You can kill every enemy without them!! Don't forget you need to wear every piece of the outfits at once or you will die in seconds! The first outfit takes the longest to grind - I suggest buying a cheap silver plated sword in Flotsam then farming endregas in the woods (at the door with the training poles) and Nekkers in the cave where you need to find corpses to prove the elf woman is a spy (if you pick up everything in the prologue you will have 1500 or more orens, it's a good start).
COMBAT
Normal enemies: cast the quen sign > jump into the battle behind an enemy > hit it with A (try to focus one enemy at a time) > roll after every 2 (later 3) hits. If you gotten hit roll away and cast quen again. This is the golden rule for every fight against normal enemies. EDIT: kill the weakest enemies first, if you decrease their number fast you'll rarely get hit and quen lvl2 or 3 paralyzes all of them.
Bosses, big creatures (like queens): The combat is basically the above except use X, it deals at least 6 times more damage and big creatures always have a hard time to turn around. If an enemy has quen go as far away as possible and run / roll around until he loose it then attack! EDIT: if you fight with a boss always attack from behind!
The Dragon: Always equip quen, don't do anything until you have it on you or you will die in one hit. If you need to wait for Vigor regeneration go as far from the dragon as possible. 90% chance she'll try to breathe fire on you but it's a slow attack so you'll get back your Vigor while she is getting ready. If you're protected (by quen) fire won't damage you and don't take away your quen! At the first phase (in the tower) roll to the left corner, she can't hit you there (EDIT: actually RARELY she can with fire). Then roll to her face hit it with X and roll back. Repeat. After you climb the top of the tower hit her face from either side with X once and roll forward (EDIT: sneak up on her on the outer rim, safer than charging her head on because only her head and claws do actually damage to you). Then turn around hit her face once with X and roll forward. Every time she try to hit you she will miss, if you do this constantly. If you get hit or need to reposition yourself then roll away, she'll either try to breath fire or try to sneak near you giving an opportunity to start the X roll, X roll again. Don't forget quen! This way you can kill her with the Oathbreaker without getting hit.
SKILLS: Forget alchemy. On magic the only Sign you ever need is the Quen (protects you from hits and damages the enemy if you upgraded) on lvl 3 as fast as possible. Training: Vigor Regeneration, Hardiness, Fortitude. Swordsmanship: Start with Footwork LVL1 ( This is very important, lvl 2 to makes you roll too much!) then go straight to the end (Whirlwind) on this route. It's better to reach the last ability as fast as possible because you can put mutagens on your skills and you'll unlock the group finishers. A group finisher always kills 3 human (wizards too!!) or 1 monster enemy instantly. I suggest only use it if you can't solve the fight otherwise because there is at least one place (after black ops achievement) - and probably more based on your skills - in the game where it is impossible (at least for me) to get through without it. EDIT: If you don't know what skills to buy with leftover points: if something raises your damage with swords or resistance for damage by 5% or 15% it's definitely a must have.
Dark mode is surprisingly easy if you played through the game once and used rolling to constantly backstab enemies and you don't even need potions, oils or changing signs!