1 Consider the most important reason you shoot your rifle. Are you hitting targets, hunting, or simply keeping the weapon for self-defense? If you shoot at targets, how far away do you normally shoot from? The primary thing to think about now is distance -- how far do you actually need to see to hit your common targets?
2 How much magnification you really need. Scopes normally come with two numbers, often something like "3-9x40." Scopes come with a range of magnifications, depending on your range. The numbers to the left of the "x" represent how much bigger the target will look than to the naked eye-- so "3x" will make the target appear three times as big in the scope. While the following magnifications are guidelines, they can help you as you start shopping.
3 Find fixed scopes for cost-effectiveness, and variability for greater range. Know that you'll pay more for high-range variable scopes. A variable scope is one you can adjust, like the 3.5-9x mentioned above -- you can adjust the zoom anywhere from 3.5 times normal size to nine times normal size. The greater this range, definietely the more versatile your actual scope will be. This, of course, comes with a much higher price tag. In general, it is best to get a smaller, more specialized scope, but spend money on higher construction. Really ask yourself how often you'll need to shoot something at 20x zoom? Unless you're hunting huge game out west, this is almost always overkill.
4 Consider usual environmental conditions and check how solid the scope feels. Your scope, at the very least, needs to be fog-proof, otherwise you'll be severely limited in your conditions. While this is generally standard, not all scopes are designed to hold up in extreme heats, moisture, humidity. If you hunt in a variety of conditions or live in extreme winter/summer environments, spend the extra money for high-end weather-proofing.
5 Need Objective Lens Diameter (OLD) to determine how much light the lens lets in. The number after your zoom length is usually the objective lens diameter, or the size of the glass at the far end of the scope, and this determines the diameter of the "main tube," which is the body of the scope. It is usually 30 millimeters or 1 inch (2.5 cm), but there are other options too.
6 Need a higher eye-relief in quality scopes. Eye relief determines how far back from the scope your head can be to see everything, and is crucial to help prevent "scope eye," when the recoil of the gun knocks the scope into your face. The higher the recoil on your gun, the more useful a higher eye-relief will be.
7 Parallax correction knob is a feature meant to add consistency and regularity to your shots. Some smaller scopes are "parallax free," but you should know what to do if your scope is not. On scopes with parallax, your cross-hairs would move slightly when you move your head, making short-range scoping difficult. A parallax knob helps with "adjustable objectives." Basically, it is a knob on the scope helping you dial in distance. If you're shooting a 200 meter away target, should set the adjustable objective to 200 yards. Now, your scope will be calibrated to give you the best shot at this distance.