First, just something little something--it seems like a truckload of pixel artists are almost confused about shading, when they should actually be concerned about the lighting, the illumination of their pieces. Anyways.
So okay, when you're doing something metallic, it's almost more important where to highlight than where to shade. But, all materials aside, when lighting, you just have to think: where is the light source, and how will it hit the object?
Metal, well in most cases, is pretty shiny and reflective, especially when compared to something like wood or carpet that will absorb most of the light.
Based on the little globes, the source of light seems to be the top left.
Just something basic to maybe push you in the right direction. Your best bet would be some real-world examples of swords, because you can simplify from them as you see artistically fit.
Good luck!