Working out a vector on a slope

[Judah Smith]

Hello again, heres one for all you physics and maths gurus out there

I understand that if a ball sits on a flat platform with a gravity of 1, pointing down at 90 degrees, the platform exerts an equal and opposite vector at -90 degrees with an equal magnitude of 1 (I think), hence the ball doesn't move. (picture 1 shows what I mean).

The question is, if the ball sits on a slope is there a way to work out the magnitude and direction of the vector representing the force exerted by the sloped platform (picture 2). I think the direction may be perpendicular to the platform (please correct me if I am wrong), but I have no idea how to work out the magitude.

Any help would be VERY appreciated. I am having a riot playing with physics for the first time, but this one has me stumped.