Thumbnail sketches are a great way to plan out an artwork. They allow you to try out a lot of ideas without committing a lot of time to each one.
Composition refers to the placement of different elements within your artwork.
Value refers to the lights and darks in your artwork.
For color scheme ideas, check out the Color Theory Resources page!
In these photos, you can see the thumbnail sketch process: starting out with lots of very different ideas; then once you find you you like, trying out some variations within that one; and finally, playing with the values and colors.
In this image, you can see some thumbnail sketches next to the final artworks. Try to see what is in the thumbnails that translated to the final art!