I want to start an Assassin character who focusses mainly on stealth/magic. Problem is I don't know what skills I should go for as I usually only play knight/paladin characters. Thanks in advance

The standard assassin class is a good place to start. Illusion can be very useful for an assassin if you want to use magic.

Race: Bosmer/ Khajiit/ Argonian
Birthsign: Thief or Shadow
Major Skills:
Light Armor
Marksman
Mysticism
Blade
Illusion
Alchemy
Sneak

Use mysticism to detect life. Illusion to hide or to frenzy/calm mobs. Alchemy for poisons. Try to use the dagger sneak attacks when you only see 1 enemy. Use your bow for sneak attacks on multiple enemies. When chaos is loose, use your blade and illusion. Use Conjuration as a minor skill as soon as you can afford the magicka cost.

Race Any

Birthsign Mage

Majors

Sneak

Blade

Light Armor

Illusion

Armorer

Any Two Fillers, possibly Mysticism or Restoration.