Depending on which edition of the book you are using, page numbers will vary, but I will give the location of the descriptions according to my version. The first sentence of chapter 1 refers to "the boy with fair hair." This is Ralph. The eighth paragraph of the chapter introduces Piggy, whose "naked crooks of his knees were plump," showing he is heavy-set and wears shorts. He is also described as shorter than Ralph, "very fat," and wearing spectacles. About five pages in, more description of Ralph is given: he is just over twelve years old with relatively broad shoulders, a mild expression, and a "golden" skin tone.
About sixteen pages in, Jack appears leading the choir boys. He is "tall, thin, and bony." He has red hair and freckles, and his face is "ugly." His eyes are blue. Shortly after Jack's introduction, Simon is described briefly as the boy who faints. When he regains consciousness, he is described as having a pallid smile, which can mean either pale or lifeless. Since he has just come out of a faint, this description doesn't describe his normal looks as much as it reveals his state. Later, around 21 pages into the chapter, his pallor has left him, and Golding says he is "a skinny, vivid little boy" who has coarse black straight hair growing thickly on his head so that it resembles a "hut." Simon is shorter than Ralph and Jack.
Lord of the Flies opens with a description of Ralph "the boy with the fair hair." Shortly after that, still on the first page (which is page 12 in my version), Piggy's description begins with his "plump" knees which continues over the page where he is described as "shorter...and very fat." The reader also learns that he has asthma (page 13) and wears "specs" (pg 14). As mentioned above, the page numbers will vary. My edition is a school edition (South African version) from 2008/ 2009. The boys discuss the fact that there are no "grown-ups" and wonder about any other boys. Ralph's description continues on page 15 with his "golden" body as compared to Piggy who is "palely and fatly naked" (pg 18).
From page 24, the other boys begin to emerge and on page 26, the reader learns that "something dark was fumbling along" the beach which foreshadows Jack and his choir's arrival. On the same page, it is clear that Jack is "the boy who controlled them," and his description on page 27 says that he is "tall, thin and bony" with red hair. He apparently has a short temper, his "light blue eyes" revealing his anger. When the boys vote for Ralph as chief, Jack's "freckles ... disappeared" (pg 30). Simon is introduced on page 29 as he "smiled pallidly" - (he is very pale) and on page 32, Simon is revealed as a "skinny, vivid little boy" with straight black hair.
The page numbers vary from edition to edition and from publisher to publisher. I can only give you approximate page numbers. Ralph is described right away in the story so his description is on the first page or two of your book. Piggy's description follows immediately. After Piggy's description, Golding spends a few pages describing the boys' actions, especially Ralph's as he plays in the water. About 12-13 pages into the chapter, after Ralph blew on the conch shell, Jack and the other choir boys enter and slowly Jack and Simon are described. The first description of Simon doesn't name him; he is the boy who, "...flopped on his face in the sand,...".