How can I draw a perpendicular plane to a plane in Geogebra?
First of all draw with GeoGebra the given plane $\alpha$, by writing its equation $2x+3y+z=12$ into the input bar.
Select then the tool "Perpendicular line" and use it to draw the line $r$ passing through the origin and perpendicular to plane $\alpha$. Create then any other point $P$ in space not on line $r$ (for instance on plane $\alpha$) and finally draw the plane passing through line $r$ and point $P$: that plane is by definition perpendicular to $\alpha$.