Juicy is the adjective.
Adjectives are describing words.  They are words that help you say how something tastes, sounds, feels, smells, or looks like.  
These are some adjectives:
juicy, delicious, sweet, sour, tasty, rotten
crunchy, loud, quiet, silent,
hot, cold, icy, frozen, steamy, soft, hard, shiny, dull, rough, smooth,slimy, squishy, prickly, furry, fuzzy
stinky, smelly, perfumed, fragrant, horrible, wonderful
lovely, gross, proud, happy,sad, nasty, bad, good,  
all colors are adjectives; red, yellow, blue, green, orange, and so on.
 
Hold an an apple with your eyes shut. Take a bite. Describe how it feels and tastes. Look at it  and describe it.  Those describing words are adjectives.

Find the noun of the sentence first. The noun in this sentence is apple.  Now ask the question "what describes the noun (apple)?"  The answer is "juicy."  The adjective always describes the noun.  Grammar can be very fun if you ask yourself the right questions.