Guild Wars 2

I’m new to thief so for pve I want to stay alive but also do a lot dmg so what is a good rune to use on my armor?

It depends, if you are are level 80 or not.
A level 80 thief has more then enough defensive skills and traits to run zerkergear, so Rune of the scholar, mesmer, thief or simular would be great. When leveling I would advise to go a lil bit more defensive. It also depends a lot of your other gear.

Personally, if you are not level 80 yet, I wouldnt worry too much and pick something that you think is usefull.

Mercury has it right. I would like to add that if you are still leveling gems work great, and if you on a budget at 80 ruby orbs are a good budget choice.