In order to write poems using similes and metaphors, one must understand what a simile and a metaphor is.
A metaphor is a comparison made between two (typically) unlike things. An example of a metaphor is: "My life is a highway." The direct comparison made is that the speaker's life is a highway.
A simile is a comparison made between tow things, but the comparison uses "like" or "as." To change the above example into a simile, one must only insert "like": My life is like a highway.
In order to write a poem about Arnold (from The Absolutely True Diary of a Part-Time Indian), be sure to include elements of his culture and heritage (given the book is about Native American Indian life).
An example of a poem beginning would be:
Arnold, brought down by water,
Sliced open like a fresh rabbit after the kill, (simile use)
Running fast, to get away,
Find his identity.
Arnold is a bear, a lion, a ... (metaphor use)
I hope this helps! Good luck!