How do you shade in a drawing??!!!?

[Liza]

All my uber awesome sketches lack soul and to give them a soul i must make them darker! or just give them a little shading but whenever i try it looks plain horrible. I don't need anything serious ( my style isn't super realistic art) just some decent shading instructions that would be found in a nicely drawn anime/manga or what would be found in Avatar: The Last Air Bender (the t.v series). Hope i'm not too demanding. ^-^