I'll assume you're thinking of a stealth assassin - so my recommendation would be Sneak, Light Armor, One-Handed, Illusion, and then maybe Destruction or Conjuration.
One-handed is undeniably a better choice for stealth than any other weapon skill. Some of the perks in Illusion go really nicely with a stealth character (Silent Casting!), and it can be really handy to mess with your enemies' AI to reduce target group size. Destruction/Conjuration will help you to deal with targets that aren't humans, or that are tricky to sneak around - as well as dispatching distant targets.
Enchanting and Smithing will also be very helpful as it'll mean you can put mage-enhancing abilities (in particular magicka regen, which is almost essential for mages) on shiny new armour.
In terms of gameplay, try not to get stuck in big groups or in close-combat with big nasty monsters, as they'll make short work of you. Let your minions and follower take the beatings. Focus on Sneak above all else; at higher levels, and with Muffle, it'll make the majority of your encounters laughably easy.