How can I achieve this "wavy" look

[Green Eyes]

Here is a pic of Ashlee Simpson. My hair is similar in length but semi-thin....how can I make this wavy look??? Please give advice and/or a tutorial. I love this look.
http://theskinnywebsite.com/site/200...tting-thinner/
It is from the July 19 V.S. Pink party. Thanks