normals for a terrain

Hi,

I'm creating a simple heightmap for my isometric game.
But I have some problems with the calculation of normals to make the lighting look nice. The problem is that I have no idea of how to do it.
A google search gives me a lot of stuff, but they all use vectors, something I don't use. And they also talks about triangles, I'm using a quad.
They talk a lot about math and I suck at it. :P

This is the code for a tile:

Type Tile

	Field y1:Float, y2:Float, y3:Float, y4:Float
	Field nx:Float[4], ny:Float[4], nz:Float[4] ' Normal for this tile

	' Resources	
	Field gold:Int, metal:Int, stone:Int	
	
	
	Method CalcNormal()
		'TODO: Implement me!
                ' I make the normal point straight up for the moment.
		For Local n = 0 Until 4
			nx[n] = 0
			ny[n] = 1
			nz[n] = 0
		Next
	EndMethod
	
	Method Render( x:Float, z:Float )
	
		glBindTexture( GL_TEXTURE_2D, Textures.grass )
		glBegin( GL_QUADS )	
			glNormal3f( nx[0],ny[0],nz[0] ) ; glTexCoord2f(0, 0) ; glVertex3f( x,y1,z )
			glNormal3f( nx[1],ny[0],nz[1] ) ; glTexCoord2f(1, 0) ; glVertex3f( x+1,y2,z )
			glNormal3f( nx[2],ny[0],nz[2] ) ; glTexCoord2f(1, 1) ; glVertex3f( x+1,y3,z+1 )
			glNormal3f( nx[3],ny[0],nz[3] ) ; glTexCoord2f(0, 1) ; glVertex3f( x,y4,z+1 )
		glEnd()
	EndMethod
	
EndType

CalcNormal() really needs some work. But I don't know how to fix it. :P
Each corner of a tile has a height (y1 to y4) and a normal nx[],ny[],nz[].
Can anyone give me some help on this?