If you get some dust, a smudge, dirt, etc., on your lens, something really bad is going to happen—that dust, or smudge, etc., is going to appear on every single photo you take with that lens. All of them. Every one! That’s why it's important to clean your lenses before you go shooting for the day, and anytime you see a little “junk” on your lens. Most of the time, you can use a simple lens cleaning cloth, but before you do that, it's best to first start by blowing any junk off the face of your lens (you can do that by just blowing with your mouth, but ideally you'd use a little hand-squeeze blower bulb), and then once any visible specks and dirt are blown away, you can clean the lens with the lens cloth by gently rubbing in a circular motion. You can get a lens cleaning kit for around $15, which includes a blower, a cleaning cloth, and particularly helpful is one that includes a LensPen, which has a little fine brush on one end, and a special cleaning tip on the other end. It works wonders.