Alliteration is when there is a repetition of using the same consonant at the beginning of several words. A great example to remember just in general is:
Sally sells sea shells down by the sea shore
^^^As you can see most of the words in the sentence start with "s"
In this poem the best line that shows alliteration is:
"of cloudless climes and starry skies"
^^^It has two sets of alliteration: cloudless and climes along with starry and skies
There are a few more examples but those only have one alliteration in the line so I didn't include them. If you would like me to just let me know in the comments
Hope this helped!
~Just a girl in love with Shawn Mendes