Forming the embouchure:

Bottom lip goes over bottom teeth, and only the line where the the lower lip meets the normal skin (which should be right over the sharp part of your teeth) will make contact with the reed. (A common mistake is positioning the reed over the flat part of the teeth.) Flex the chin down (and perhaps think slightly forward) while putting the sharp edge of the lower teeth into the place just under the inside of the lower lip. Do not let any of the flesh of the chin touch the reed.

If this all sounds confusing, think of the bottom lip going up while the chin goes down. The corners of your mouth should be firmly sealed, and try to avoid a "smiley" looking embouchure. Other tips: Think about lengthening the total area between the tip of the nose and the bottom of the chin. This area between the nose and the top lip (the "filtrum") should be lengthened, as there should be some embouchure support and strength coming down from above, not just from below.