Like people already stated, Combat Swords is the way to go. I agree with the spec Bishop already linked. The first few specs in Assassination often seem tempting to pic, but ultimately there's simply not as good as the options you have in Combat. Like stated before, just fill out Combat first, then spec into Assassination afterwards.
For example, at lvl 30 you'd be specced into the following:
http://www.wowprovider.com/?talent=1121 ... 2305501d5yUntil level 30+ when you start actually speccing into Swords, you can obviously use any decent weapons you come across regardless of weapon class, since you wouldn't get the extra Sword spec bonus anyway.
However afterwards, there's a couple of weapons (from quests) you really do not want to miss out on:
Horde -
Sword of Omen lvl 33 required, lvl ~37 recommended.
Alliance -
Sword of Serenity lvl 33 required, lvl ~37 recommended.
Both factions -
Vanquisher's Sword lvl 37 required.
Both factions -
Thrash Blade lvl 45 required.