Find a finite Gröbner basis for the ideal

I need to find a finite Gröbner basis for the ideal $$I\subseteq\mathbb{R}[x,y,z]\ \ (x>y>z)$$ where $$I=\{f\in\mathbb{R}[x,y,z]\ |\ f(a,-a,2)=0\ \forall a\in\mathbb{R}\}.$$ The thing is I have no idea how to start the solution (because I cannot really understand what is a finite Gröbner basis). Any help would be very helpful!