Yeah, that's too bad that you acted without an understanding on how disenchanting works. I'd recommend you get an addon like enchantrix (if it still works), to help you select items to DE.
For the most part, green trash items disenchant into dust, essence, or shard. If you use an addon like enchantrix, it comes with a database of old-world greens, so when you mouseover an item, it will give you a set of probabilities, and an average number of products. This means that if you disenchant an item, you have an X% chance to get dust, Y% to get essence, and Z% to get shards (with X+Y+Z~=100). The average number of each product that is listed is exactly what it sounds like; the number of products of individual DEs may vary, but if you DE a statistically significant quantity, you should find the averages to work out. Typically, weapons have greater chance to DE into essence, and armor into dust.
This post is not exactly all-encompassing, but its a pretty good start.