How can I write a 14-line poem about My Side of the Mountain?
A 14-line poem means that your poem is going to be the length of a standard sonnet. However, if the line requirement is the only requirement (besides topic), then you do not have to follow any of the other sonnet rules. That is going to make your job easier. If you have to follow typical sonnet rules then I recommend going with a Shakespearean sonnet, which has 14 lines with 10 syllables per line. I wouldn't worry about going for perfect iambic feet, because chances are it will naturally fall close to it (iambic meter is really close to natural spoken language). It should rhyme as follows: ABAB CDCD EFEF GG.
That seems like a daunting task, so break the 14 lines into 3 different 4-line stanzas. Then wrap it up with a 2 line couplet. I don't recommend trying to condense the entire plot of the book into 14 lines. If you can do that, you're awesome. I would make the poem a post-adventure reflection. Stanza 1 could be about his initial optimism about the trip. Stanza 2 could be about his fears or concerns. Stanza 3 could be about his gaining confidence and successes. And the remaining couplet would be his final thoughts/recommendation to readers, kind of like a concluding sentence.
That stanza format should work fine even if you don't have to have rhyme and meter. If that's the case, I recommend you write it more like an emotional journal reflection. Use lots of adjectives and throw in some similes and metaphors. That's an oversimplification of what poetry is . . . but it's true too.
Go back and look at the requirements. See if the poem needs to rhyme or have certain rhythm. Follow those guidelines. From there, you could approach it as a summary of the book, using 4-5 lines for the beginning, middle, and end. You could also choose to focus on one character and write about one scene from the book from either 3rd person or even 1st person as if from the point if view of one of the characters from the point in the book that you choose.