About the Frechet derivative of a functional

Question

How can I compute the Frechet derivative of the functional $$ I(u)=\frac{1}{2}\int_{\Omega} \vert \nabla u\vert ^2\ dx \ + \ \int_{\Omega}\left[1-|u|^2\right]^2\ dx$$ for $u$ in a functional space with norm $\Vert u \Vert ^2=\left( \int_{\Omega} \vert \nabla u\vert ^2\ dx + \int_{\Omega} \vert u\vert ^2\ dx \right)$. I still haven't study the Frechet derivative but I need it as soon as posible. Any advice, hint or a good reference to sea this will be appreciated. Thanks in avance.