You cannot disenchant ALL green items, just most of them. Any green, or blue or (god forbid) purple gear can be disenchanted, as far as I know, but gems, bags and potions cannot.
No, you cannot disenchant essences or dusts or shards of any kind.
I have tailoring and enchanting at 300 each. Please take my advice and forget that you have enchanting at all until:
a) you reach a skill of 80 from just disenchanting things (easiest way to do it)
b) You've got cash to burn (preferably post mount, but not necessarily)
c) You've got cash to burn.
Getting from 80 + is really hard, because no one wants +5 health on a chest piece, or +1 stamina on a bracer. They jsut won't pay for it. So disenchant for monies, then later on enchant your own stuf over and over and get some higher (like 225+) skill. That is the fun part.
Enjoy!