Now that that's out of the way...
Ninja is obviously the best bet.
Rogue/Hexblade could kind of work, with the hexblade's curses hitting the opponent. Look into using poisons, too, of course.
As someone who loves Spellthieves, I hate to admit that a spellthief/shadowdancer is a bad idea. If you go Spellthief, it's spellthief only (unless you take levels in sorcerer or wizard, and take the master spellthief feat from Complete Scoundrel...)
Really, though... a plain old Rogue who goes into Shadowdancer probably fits what you want the best... and since you're new to D&D, it might just be the most enjoyable combination. High Sneak Attack damage (which is what you want), coupled with the Dancer's hiding abilities.
As for race... Human is great, although Elf works well for the concept too (since they can see in the dark, and have a good dexterity). Feats should be dictated by personal choice, although I think the big three are all from the PHB - Improved Initiative (so you can shoot first, thus getting sneak attack damage), Combat Expertise (so that if you're cornered, you can stay alive long enough until someone rescues you), and Improved Feint (so you can make sneak attacks during combat!).
Make sure you put max ranks in Bluff and Tumble, (so you can get your sneak attacks by feinting, and so you can avoid Attacks of Opportunity) and then you'll want high ranks in Hide and Move Silently (obviously). The rest is up to personal preference.