I'm going to ignore all the other posts in this thread and respond solely to the OP.
An A/W with Moebius Death Blossom (insert lead/offhands of your choice...anything works really) and Save Yourselves! OR an A/D with Wounding Strike, Way of the Master and other critical hit enhancing options with a scythe are your best options for damage dealing in regular PvE. As a starting Assassin, I highly recommend being A/W, and picking up the other secondaries as you can, /E, /Me, /D & /N being priorities, in that order. Elites to pick up are Moebius Strike, Shadow Form, Assassin's Promise in that order, and then others to taste. I like to bring Temple Strike, Palm Strike & Flashing Blades on my Sin when I am really really reallllllly bored so you might want to look into that. Also consider capping stuff like Wounding Strike and a couple of the warrior elites (Earthshaker on a Crit Agility Sin is insane fun), but the core builds will get you through 95% of the game's PvE.
PS: Forgot to add, take your Assassin through EoTN AS SOON AS POSSIBLE. The PvE skills make your job a lot easier as an up and coming sin. Max out your Ebon Vanguard, Asura, Delver & Norn ranks (in that order) for epic PvE rapeage.