Adjusting brightness
It has happened before that some of the photos that I have taken turn up dark when I upload them in my computer. Instead of cursing myself for being too lazy to see to the lighting, I simply edit said photos and voila! Problem solved!
What I do is use the gamma control in my image editor. Most image editors have this feature in their Image or Color menus. This tool actually retains the already bright parts of the image while it brightens the dark ones. There is no prescribed setting. I suggest that you experiment with this tool first to learn how to control the right brightness for your photos.