I have started working on my pixel art skills by going through some of the tutorials available at http://www.pixeltutorial.cjb.net/. So far all I've got is four 16x16 grass tiles, but that's a start.
Here's all four tiles in a regular pattern. Not too bad for three shades of green. It's not perfect; you can still see the tiling somewhat since the four tiles are placed out in order, but if I randomise those four tiles and stick in a few flowers or rocks I think it will look better.
This took me a bit longer that I'd hoped, since I spent an hour or two trying to find an editor that is suitable for pixel work. The GIMP wasn't very useful at all, and Microsoft Paint doesn't have enough features to not be annoying. Since that glitch wiped my hard drive a couple of months back I had to trawl through my backup archives to find all the other editors I had collected, and finally settled on the free version of GraphicsGale. It seems more suited for animation than tile work, but it works okay once you get used to it. I think if I get serious with working with tiles I'll need to find a more full featured tile editor, or make one up myself with all the features I want. Are there any suggestions for good, cheap pixel art editors that work well with tiles?