I'm new to Elder Scrolls and I'm starting a new character and i need some help.

Obviously one steals stuff and one kills people but in terms of weapons, armor, abilities, perks, etc. What is the difference between playing a thief and an assassin?

There's really not a large difference. People tend to have their Assassins be theifs anyway and it's usually a toss up of whether they want to make an Archer or an Assassin.

u can easily be both, there's not much difference apart from what you mentioned. You can at higher levels be both archer and one-handed assassin....80 perks total

If you are a thief you dont spend your money on perks so you can one-shot everyone, you spend your perkpoints so you become very hard to detect when you sneak around. Both "classes" have light armor but a thief would be have more enchanted armor that boosts sneak, lockpicking, speech, pickpocketing while an assassin would get armor that helps archery, sneaking, one hand weopons.
My tip to syou would be to play both! You can easily make an excellent thief/assassin in the game. Go for both the dark brotherhod and the thieves guild

Assassin type characters are alot of fun in Elder Scrolls games. You're making the right choice. I always have a Warrior and an Assassin.

Are you reading my mind...? I'm about to finish my Warrior and start an Assassin O_o... Anyway, Thieves are generally in it for the money and loot verses Assassins are in it for the kill. Thats the general difference, in terms of gameplay, they're very similar.

Heh, I always have to have a Warrior and an Assassin! Makes for the most fun!

Become a thief assassin if your not sure which one you want to play as.

Or, at the cost of some slightly different perk selections and some level ups in magicka, be a thief-mage-assassin!

Add some illusion to your stealth character to augment your sneaking skills when moving in for the kill or swiping items or throw out some fury magicka at an unsuspecting group of bandits and sit back while they tear each other apart! Or you can take Alteration in place of Light Armor or in combo with Light Armor for some shielding spells in case of a head-on confrontation or to paralyze your victims!

Anything is possible when your thief is a mage and not a fighter. I'm on a horse. *Old Spice whistle*