There are many forms of literary devices, and almost anything you point out about the way something is written could be a literary device of some kind.
The main literary devices are: metaphor, similie, personification...
There are also devices of sentence structure: repetition, gradualism, parallelism...
Then there are literary devices that relate to the sound of the writing: onomatopoeia, alliteration, rhyme...
However, if you find something interesting about the way it is written, then you should copy out the quotation and look at why it is attracting your notice. Why you noticed it is probably related to why it's a literary device of some kind. Once you have got the quote, then you just need to be able to describe the literary device in your own words.
Katherine Paterson's Bridge to Terabithia contains numerous literary devices. In fact, the novel opens with one: "Ba-room, ba-room" is an onomatopoeia (a word which mirrors the sound it makes).
Soon after, the text contains a simile (a comparison between to typically dissimilar things, using "like" or "as" to make the comparison). Jess Aarons decides not to put a shirt on because he will be "hot as popping grease" when he runs. Another simile soon follows when Jess's mother (when mad) is compared to fruit flies in a jar.
Repetition can be seen when the word "fastest" is repeated four times in one sentence: "He had to be the fastest--not one of the fastest or next to the fastest, but the fastest." This repetition illustrates how important being the fastest is to Jess.
Personification (the giving of human characteristics to non-human/non-living things) is found when the narrator states that Jess's house would moan and screech as people walked about. This "moaning" would be a human characteristic given to the house.