Remember that similes and metaphors are both examples of figurative language where one thing is compared to something else, normally something that isn't normally associated with the first thing. The only difference is that with a simile this comparison is indicated with the words "like" or "as," whereas with metaphors the assertion is direct without these words.
A good example of a simile comes from the chapter entitled "Escape" on p. 97 of the text. Note how Melinda describes how it feels not going to school:
It's like living in an MTV video--not with the stupid costumes, but wearing the butt-strutting, I-do-what-I-want additood.
Here Melinda is using the simile to describe the kind of confidence that not going to school gives her, and there is an obvious parallel to her meek and timid attitude at school. Note the word "like" indicating that this is a simile.
A good example of a metaphor comes at the very end of the book, after Melinda submits her picture of the tree:
The tears dissolve the last block of ice in my throat. I feel the frozen stillness melt down through the inside of me, dripping shards of ice that vanish in a puddle of sunlight on the stained floor. Words float up.
Note how Melinda uses the metaphor to compare the "block of ice" to the emotional damage that was done to her as a result of the rape. This is a metaphor that is used at various points in the novel, and in this quote the reader sees the ending of the "ice" as it melts and she is able to finally speak again.