First of all: Awesome to see a real artist try his hand on new content here.
First you must understand that graphics and tilesets are different. Only creatures use sprites. You have spriters for adult creatures, child creatures, baby creatures and undead creatures. Also special sprites for war-trained and hunting-trained ones. Thats it.
If you make sprites for intelligent creatures, that are part of a civilization, the sprites work by profession. So you have mason, miner, swordsdwarf, and so forth. Nobles also get special sprites.
What you cant do, is that shield of yours. Shields are items of the type SHIELD. Sounds silly, but thats how it is. Like a "large serrated disc" is a item of the type TRAPCOMP. They use a tile from the tileset, which is hardcoded. You can change how the tile looks, but you cant change their tilenumber. Only a few things allow this, like grasses, plants, trees and inorganics. Items like furniture, armor, weapons, etc are hardcoded.
A big problem is that Toady One mapped many graphics into a single tile. For example the tile that is used for BLOCKS is also the tree-top, when seen from a zlevel above. Or broken ammo uses the same tile as ASH. Or shields, armors, traousers, shirts, pants, gloves and helmets all share the same equipment-tile.
This means that making a tileset is a lot harder than "just" designed the actual graphics. It also has transparency, and foreground/background color that you can do tricks with. I for example use letters for vermin. GREY/BLACK looks like a grey letter on black background, but BLACK/GREEN looks like a green snake on black background. This works by using grey for the letter, a solid black for the background and a 100% transparent snake painted around the letter.
The best utility to test how this looks ingame is workshop draw, which can load different tilesets and show colors.
I was working on a 64x64 or 48x48 tileset that has been abandoned for now (http://www.bay12forums.com/smf/index.php?topic=123604.0
) and I made the MDF tileset, the default tileset for the Masterwork mod.
If you want to do a tileset, I would be so happy with a 48x48 one. I would help as much as I can, and add a lot of creature sprites. Roses just brought that topic up 2 days ago... guess there is some interest for it.
EDIT: I know this a bit incoherent. ^^ Anyway, the tilesets are in data/art, in case you didnt knew this. The wiki has some great pages about it,, like this one http://dwarffortresswiki.org/index.php/DF2012:Tilesets
Colorschemes also play a role. The resident expert is Vherid.