An Assassin is a specialist, they rely more on skill than attack and have very little in the way of defense. They use speed and debuffs to eliminate their targets.
Attributes:
Dex - This is your main stat, should be the highest.
Str - This is your secondary stat, needed to equip weapons and armor.
Vit - This increases your HP and HP recovery rate, some people put into it to survive easier.
Mag - This is useless for an assassin.
Builds:
Pure Dex: This is the best build if you can afford it and live. Later on when getting equips with str boosts on them you may not need to add str for some levels or at all.
4 Dex 1 Str every level
Hybrid: This is the most common build, the easiest. Adding some Vit so you are better able to survive. Most people that use this build cap their Vit low, from 20-40.
3 Dex, 1 Str, 1 Vit every level
OR
7 Dex, 2 Str, 1 Vit every two levels
Skills:
First Attack Tree:
Puncture Wound - Very nice bleed damage, max after your main attacks.
Wolf Emblem - Increases crit, lasts 30 seconds.
Knife Throw - Low damage, but it's a ranged attack without using a bow, useful for ranged mobs when they try to run away.
Tackling Slash - Average damage, long cooldown, stuns target... gives you chi.
Chill of the Deep - Increases your damage, but lowers your attack speed. Lasts 10 minutes. 1-3 points for aggro control at bosses, some say max.
Rising Dragon Strike - Good damage, low cooldown, gives you chi, 100% accurate. It's pretty good. Level slowly as you get extra points.
Inner Harmony - A difference of 45 chi gain from lv1 to lv10, and a difference of 76mp from lv1 to lv10. Up to you if you think the MP cost is worth it. If demon, get lv11. Sage is personal preference.
Stealth Tree:
Shadow Walk - Good for stealth, but it's MP heavy, not worth many points early on. But you want to max it by early 80s for FC.
Focused Mind - Good defense wise, if that's what you like. Optional past level one.
Shadow Escape - Helps get away from a mob, not worth many points. Max it by early 80s for FC.
Bloodpaint - One point early on, only adds time until level 11.
Deaden Nerves - Can save you sometimes from things that would have killed you. One point for early on, doesn't add anything until lv11.
Tidal Protection - Useful against monsters that cast negative effects, other than that, mostly useless. 27.5% at lv1, 50% at lv10.
Second Attack Tree:
Twin Strike - Quickly overpowered by your other skills, don't waste your time.
Slipstream Strike - Very powerful early in the game, max asap.
Raving Slash - Low damage but slows your enemy, up to you, but other skills do more damage.
Deep Sting - Mostly only used to sleep mobs or players in PvP/PK.
Rib Strike - This is your BEST attack currently, max this asap.
Throatcut - A pretty good attack, seals opponents. Level after main attacks.
Earthen Rift - Very good damage, one of only two AoE skills, max it.
Subsea Strike - Your best AoE, increases damage. Must have for attackers.
Headhunt - One of your strongest skills 1v1, max asap.
Power Dash - Really good, gives you high crit rate, max after attacks.
Buff Tree:
Windpush - Just increases speed and not by much at that, not worth points early on. But you will probably want to max it later for speed.
Shadow Jump - Pretty good at level one, increase if you like it and have extra points. Max later on for more range.
Maze Steps - Good enough at level one. Max later on for more speed increase.
Shadow Teleport - Long cooldown, good enough at level one. Max later on for more range.
Others:
Tide Form - Can only use normal attack and must be underwater. Worth a point to help defense underwater, more is optional.
Cat-Like Tread - Increases your stealth. Should put into this around 80 for FC and earlier if you do PvP/PK.
Dagger Devotion - Passively increases your damage, max asap.
Sharp Observer - None, monsters don't hide. Only necessary for PvP/PK against other Assassins and now Archers have a stealth skill also.
Condensed Thorn - Good for extra water damage, but expensive to get book. Optional.
Healing Trance - Very good and only healing skill, book is expensive but get if you can.
Spell Cutter - Doesn't seem like really good damage and book is expensive.
Blood Frenzy - Pretty good for healing yourself, but again book is expensive and Healing Trance seems better.
Skill Upgrading Priority
Group A: Top Priority, Must Have Up to Date at all Cost
Puncture Wounds (lvl 3)
Slipstream Strike (lvl 6)
Rib Strike (lvl 29)
Dagger Devotion (lvl 29)
Subsea Strike (lvl 59)
Group B: High Priority, Keep up to date, but not at the cost of Group A
Tackling Slash (lvl 29)
Focused Mind (lvl 19)
Rising Dragon Strike (lvl 49)
Wolf Emblem (lvl 9)
Group C: Optional, get these based off personal preference only.
Cat-like Thread (lvl 29)
Sharp Observer (lvl 29)
Deaden Nerves (lvl 39)
Shadow Jump (lvl 29)
Deep Sting (lvl 19)
Throatcut (lvl 39)
Group D: Low Priority, get it only when you have enough to spare
Tidal Protection (lvl 59)
Shadow Walk (lvl 13)
Shadow Escape (lvl 29)
Knife Throw (lvl 19)