I am assuming you will be writing a Shakespearean sonnet, which has 14 lines with 10 syllables for each line. The rhyming scheme must be "abab-cdcd-efef-gg": in other words, the first and third, second and fourth, fifth and seventh lines, etc. must rhyme, as must the last two lines. With these structural concerns out of the way, let us look at Friar Laurence as a character. As a holy man, he is an important confidant for both Romeo and Juliet. He takes an active and sincere interest in both of their lives, chiding Romeo for his lovelorn nature and commiserating with Juliet upon Romeo's banishment. He agrees to marry the two lovers because he hopes that their marriage will bring about a truce between their two feuding families. Once Juliet's engagement to Paris is determined by Lord Capulet, he concocts a scheme to get her out of it, and to reunite the couple. While this scheme is obviously a bit farfetched, dependent on a number of factors for success, the fact remains that he undertakes it in the interest of Romeo and Juliet themselves. So a sonnet about Friar Laurence should include a discussion of these characteristics, and probably should consider as well that the Friar, in all of his actions, wise or unwise, was always struggling against fate.