Guild Wars 2

So, I purchased Gems for the first time today, and I wanted to purchase an Amor Skin for my Necromancer, my question is it a one time use? Or will I be able to use it when I get new pieces of armor?

When you get the item, you can put the skin on your curretn armor. If you want to put it on other armorsets in the future you need to use a transmutation stone (till level 79, basis, afterwards fine). You get a lot if you complete area’s to 100% but if thats not your thing, you can get them as well for gems.

So I can buy the Armor Skin, in this case the Profane since it’s the only one I can use, and use it on my current armor, and use Transmutation Stones to apply it to other armors as well?

Do I need to keep my current armor that I first applied the skin to in order to do this? Sorry, I’ve never bought Armor skins before =)

You almost got it.
A transmutation stone gives you the possibility to mix aspects of 2 armor pieces, the outcome is always one piece (so you put the skin of the old armor on the stats of the new armor, the old armorpiece will be gone). Best is to watch this video where it is explained with the interface:
http://www.youtube.com/watch?v=qektXiOsmtg

Armor A (transmute using Profane) to Profane Armor A, Get Armor B (transmute using Profane Armor A) Profane Armor B

So yes, you just retransmute using the old piece and keep passing the skin on up.