I made my map with Single Player Commands, but that mod hasn't kept up with the Minecraft updates. I don't really like MCedit, but I haven't used it enough (I don't like the movement controls, but there is probably a way around them).
1. Use an editor, once you find one watch youtube tutorials and learn how to use it. If you don't like it, don't use it. But master the one you like.
2. What ever inspires you. If nothing inspires you play a few custom maps (good ones and bad ones) hopefully inspirations takes shape.
3. I made my map without a texture pack. But maybe just use your favourite texture pack, unless there was a specific theme you were going for.