So as always... I found the partial derivative with respect to $x$ and $y$ of $f(x,y)$ which gave me:
So I wasn't too sure what to do next, but I set $f_x = 0$:
$0 = -2x$
And I got stuck again. How do I continue AND prove that $f(x,y)$ at $(0,0)$ has a critical point but is not max/min value? Thanks!