1*.Consider Aperture and magnification: the two most important qualities of binocular selection. Aperture refers to the diameter of the lenses. The larger the aperture, the more light the binoculars gather and the better you will be able to see faint objects. Magnification refers to how large the image appears and is limited by the aperture size. Be wary of binoculars that are advertised by magnification only. High magnification with a small aperture is nothing but bad marketing.

(Magnification)x(Aperture in millimeters) is how aperture and magnification are represented. For example, binoculars that are 15 x 70 have an aperture of 70 mm and magnify the image by 15 times. These are a great starting pair for astronomy binoculars.
2*.Consider the weight. Binoculars with large apertures are quite heavy. 20 x 80 binoculars may weigh over 5 pounds! While larger may seem better, if they are too large to use, they are no good.
3*.Consider the cost. The price of binoculars does not increase proportionally to size. Larger binoculars become expensive quickly. You can get a good pair of beginner's binoculars for around 60-70 USD.
4*.Consider a monopod or tripod. With heavy binoculars with high magnification, the image will move around a lot if they are hand held. Binoculars can be mounted to a monopod or a tripod. This will steady the image. A monopod is nice because they are extremely portable, steady the image a bit and give your arms a break. If you want to use a mono- or tripod, make sure the binoculars have a sturdy attachment point.