Adding the cat

The Lasso tool is on the tool panel

Set the feather to 1 in the Properties window

Using either the Lasso or Polygon Lasso tool outline the entire cat

Invert selection and delete the green background and it will look something like this

This how your image will look when you first paste the cat on the page

The scale tool is found right below the select arrow.  This tool allows you to rotate your image 

Your final image should look something like this.