Most people have figured out how to make changes that affect their digital images, such as changing the brightness and contrast or automatically adjusting the color.
But what do you do when only part of your image has a problem? Most image editing software comes with selection tools that you use to select just part of an image. The tool you use depends a lot on what you want to select. I'll use Photoshop as an example, but virtually all image editing programs have similar tools.
In Photoshop, for example, you have rectangular and elliptical tools. They do what you'd expect; they let you select a rectangular or elliptical area. However, as you've probably noticed, most items in your digital pictures aren't nice precise shapes like squares and circles. By holding down the Shift key, you can use these selection tools additively. In other words, the selections "glue" into each other to make one larger oddly shaped selection.
The Lasso tools are another way to select unusually shaped items. The regular Lasso tool lets you draw freehand to select an area. Just click and drag and the area is selected. Many times when you are selecting, it helps to zoom in to get a better look at where the edges really are. With the Polygonal Lasso tool, you click to set each point in a polygon. The Magnetic Lasso tool lets you set points, but the lines aren't straight. All of these tools have innumerable keyboard combinations that let you change how they work. They also take quite a bit of experimentation before you get a real feel for how to use them.
If the problematic item in your image is a solid color, you may want to try out the Magic Wand tool rather than laboriously tracing the item. This tool lets you select a consistently colored area, such as an expanse of blue sky. You click to select a color and use the numerical settings to change the color range (or "tolerance") the magic wand selects. Higher numbers cause the tool to select more shades of colors.
When you are selecting items, a good command to keep in mind is Photoshop's "deselect" command. For example, if you don't get the tolerance set correctly, sometimes the Magic Wand selects way, way more pixels than you want. You can click anywhere outside the selection to deselect the areas, choose Select|Deselect from the menu, or press Ctrl+D.