Sneak, Pickpocket, One handed, Conjuration, Illusion should be considered your main perk trees, also consider archery if you get Bound Bow.
http://www.uesp.net/wiki/Skyrim:Sneak
Sneak:Assasins Blade - right side of tree, ignore the left.
http://www.uesp.net/wiki/Skyrim:Pickpocket
Pickpocket:Poisoner, not much else is useful for assassin unless you're also a thief.
http://www.uesp.net/wiki/Skyrim:One-handed
One Handed:Bladesman, self explanatory.
http://www.uesp.net/wiki/Skyrim:Conjuration
Conjuratoin:Mystic Binding, this is an early perk so you can save having to pump wasted points.
http://www.uesp.net/wiki/Skyrim:Illusion
Illusion:Quiet Casting, unfortuantely quite high up the tree so this is really optional.