I know women who shy away from eyeliner because it can be a complete disaster. This fear is warranted; eyeliner is transforming. In just a few simple strokes, a face can go from sweet 16-year-old to questionable streetwalker. It can however, with a few well placed strokes, also go the other way to polished sophistication. If you prefer the latter option, it is by no means an impossible task.
Before focusing on eyeliner, I'll outline the basic objective of everyday makeup. The goal of makeup: enhance rather than destroy your features. Eyeliner's purpose is the same. I've seen women (and men scarily enough) with eyeliner that makes it virtually impossible to see the real face. It's supposed to draw attention to the eyes, but not so much that you fail to see the rest of the face. You want it to look as natural as possible. An oxymoron of sorts, you don't want it to be noticeable. Your eyes should be enhanced enough so that on a first glance, people notice the eyes rather than the makeup.
Choose a color that's not too overbearing and that matches your skin tone. If you're a blond Caucasian, don't opt for a jet-black liner. If you're African American, don't choose a light twiggy color that won't show up. You have natural coloring so go with it! For most people a mid-brown tone works well; it shows up enough to make a difference, but doesn't take over the eyes. Brown eyeliner can also greatly enhance eye color—with the right shade of chocolate eyeliner, brown eyes can pop! Regarding blues and greens, it's not completely forbidden—just remember to keep it looking as natural as possible. (Combining colored eyeliner with a natural brown is a good way to keep colors balanced).
While it is called eyeliner, you don't want to line your entire eye. Rather than complementing the eye, all-over eyeliner will shrink it. Line your eyes about 3/4 of the way in on the bottom and you can go a bit farther on the top. Apply with a moderate stroke, keeping it on the thin side. Too thick a stroke will invariably lead to a tipsy Jack Sparrow look and too thin won't enhance the eye at all. If the thought of a using an eyeliner pencil makes you uneasy or nervous you can always opt for dabbing a little bit of a dark brown eye shadow on the corners of your eye with a thin angled brush to achieve a more defined look.
The core rule of all everyday makeup, including eyeliner, is enhancement, not defacement. Keep it natural. You can now lean toward the side of refinement rather than raunchy, style as opposed to sleazy.