Well. You need to think about what your making. If your trying to make a human face, you don't want to have blue or green tents in the colors your picking, you want yellowish tents. Also, you need to look for colors that blend nicely together. Look at some of the awsome pixelers here, find a picture you like, and try to match the colors. Don't use the eye dropped and select all of the colors, but try to do it by double clicking a color, and trying to find a color as close to it as you can. Then, after you get the colors as close as you can, use the colors you used, use the fill tool, keep a copy of the original picture, then paste the same picture next to it, and fill in your colors.

If you have any other questions, just ask.