So I've played around with Artificing a bit, but can't seem to make anything useful. I've looked around a bit, and found some neat schematic guides, but I don't know how to learn these schematics, and haven't seen anything in game that suggests how I might go about it. Where do I start, where do I find more schematics?
There's a few ways to learn more patterns for your Crew Skills.
The first, and most obvious, is to Learn them from your trainer. As your skill improves, your trainer will have new schematics availiable to you for a small fee. Generally speaking, items that require a higher skill to create will require better materials, be more powerful, require a higher level character to use, and cost more to learn to craft.
The second, and nearly as important, is Reverse Engineering. Essentially, by deconstructing the items you can make, you'll learn to make improved versions. See this excellent question for more details on exactly how this works, but all you really need to know is that if an item doesn't seem great, you can probably reverse engineer it to learn how to make something better.
The third and final way, is to find a schematic and learn it. In general, items that you can learn to craft in this manner can only be learned in this manner. Schematics can be found in a variety of places, including as drops from mobs (including especially rare ones from special bosses), in lockboxes acquired as quest rewards or from Commendation Vendors, world drops, through Mission Skills (I believe Artificing patterns come from Treasure Hunting primarily, though I may be wrong on this one), and if all else fails, you can buy the patterns from somebody else that found them but can't use them over the GTN.
You learn green recipes from the trainer. You learn blue recipes from reverse engineering greens. You learn purple recipes from reverse engineering blues.
On the galactic trade market, don't expect many people to buy greens. Blue should be the cheap tier on the market. Purple, the deluxe tier (costing 4 or more times as much as blue).
As far as "useful"...
Hilts, crystals and mods are useful if you're using an Orange lightsaber. When you upgrade the hilt, the damage goes up!
There could be a market for cheap off-hand items. I see a bunch listed on my server from treasure-hunting rewards... however they're listed crazy high (2k for a level 9 item). It only takes ~300 credits to make a blue level 9 off-hand item with artificing.
Artificers can make Lightsabers, Gormak Crystals (for the Nightmare Pilgrim boss), Relics, Generator/Focus offhands, and dyes. The mods they make are Enhancements, Crystals, and Hilts (not all mods).
Since the Explosive Conflict operation was introduced in Patch 1.2, Artificers can make end-game gear via Reverse Engineering. Enhancements and hilts from Black Hole/Campaign (level 61, tier 26) and Hazmat/Dread Guard (level 63, tier 27) gear can be ripped out and RE'd for a chance at learning the schematic. As well, Campaign and Dread Guard relics from Flashpoints/Operations and the Classic Commendations vendor can be RE'd for a chance at the schematic.
Since the level cap increase (Patch 2.0), Black Market/Arkanian mods (level 69, tier 30) can be reversed. Verpine/Underworld (level 72, tier 31) mods can be reversed since Patch 2.4. The corresponding relics from both vendors can only be RE'd for schematic if they were a drop from an Operation; Relics bought with Commendations cannot be learned.
A few low-level crystal schematics come from Investigation, and four Lightsaber schematics come from Treasure Hunting. The Magenta crystal schematics mostly come from world bosses as another answer notes, with Orange crystal schematics also dropping from the Primal Destroyer. Some Rakata (level 58, tier 25) enhancement schematics drop rarely in Operations. Finally, the Gormak Crystal schematic is available from the Daily vendor; that's it apart from what you learn from the artifice trainer.