This is book number 1 in the Animorphs (Prebound) series.
The Earth is being invaded, but no one knows about it. When Jake, Rachel, Tobias, Cassie, and Marco stumble upon a downed alien spaceship and its dying pilot, they're given an incredible power -- they can transform into any animal they touch. With it, they become Animorphs, the unlikely champions in a secret war for the planet. And the enemies they're fighting could be anyone, even the people closest to them. So begins K.A. Applegate's epic series about five normal kids with a limitless amount of forms and abilities.