There are a fair number of assassin builds you can go with, as the prereqs for the assassin class are pretty basic (just need to be evil and have 8 ranks in Hide and Move Silently). Three common starter classes to build an assassin off of are Rogue, Ranger, and Monk.
Rogue will give you the greatest versatility, with plenty of skill points for opening locks, detecting and disarming traps, and social skills, but the downside is that your combat abilities won't be nearly as good- you'll be relying almost entirely on sneak attacks for your big damage, which can make things difficult when dealing with enemies immune to sneak attacks (such as undead, which you'll be seeing plenty of in the OC). Rogues also get some nice feats available to them starting at level 10, such as Improved Evasion (which will let you often completely avoid damage from spells that allow a reflex save).
Ranger gives you less skill points to play with, but greater combat abilities- you'll get Ambidexterity at level 1 (so you can duel-wield effectively), and Improved Two-Weapon fighting when you take your 9th Ranger level. The animal companion you get can also be useful for providing some extra distraction, letting you get in sneak attacks easier. Favored enemies can also provide a nice bit of extra damage against enemies that you otherwise might have some trouble dealing with (such as the undead).
Monk has less offensive power than a Ranger build, but greater survivability. With high dex and the monk abilities you can get a good AC, letting you go toe-to-toe with enemies that you can't take down from behind. Monks also get in a large number of attacks (especially with Flurry of Blows), which can be absolutely devastating if you open with a flurry of sneak/death attacks. Get some gloves or kamas with elemental damage and you can quickly stack up large amounts of damage just on normal attacks (even if the individual damage from the attacks is small, it can very quickly add up).
With any of these choices you'll need to go 4 levels in your starter class, then you can start taking Assassin levels. Whether you want to go strictly assassin at that point until you hit 10 levels in it, or do a mix of assassin levels and levels in your starter class is up to you, and will probably depend on what your starter class is (and which levels that class gets various feats you may be interested in).
