I have been googling this for about two days. I have got to a point where I can find a direction based on my mouse position on screen, but it seems to not be affected by the yaw or pitch of my camera.

        IntBuffer viewport = ByteBuffer.allocateDirect((Integer.SIZE/8)*16).order(ByteOrder.nativeOrder()).asIntBuffer();
        FloatBuffer modelview = ByteBuffer.allocateDirect((Float.SIZE/8)*16).order(ByteOrder.nativeOrder()).asFloatBuffer();
        FloatBuffer projection = ByteBuffer.allocateDirect((Float.SIZE/8)*16).order(ByteOrder.nativeOrder()).asFloatBuffer();
        FloatBuffer pickingRayBuffer = ByteBuffer.allocateDirect((Float.SIZE/8)*3).order(ByteOrder.nativeOrder()).asFloatBuffer();
       
        GL11.glGetFloat(GL11.GL_MODELVIEW_MATRIX, modelview);
        GL11.glGetFloat(GL11.GL_PROJECTION_MATRIX, projection);
        GL11.glGetInteger(GL11.GL_VIEWPORT, viewport);
        float winX = (float) Mouse.getX();
        // convert window coordinates to opengl coordinates (top left to bottom left for (0,0)
        float winY = (float) viewport.get(3) -(float) Mouse.getX();
       
        // now unproject this to get the  vector in to the screen
        // take the frustrm and unproject in to the screen
        // frustrum has a near plane and a far plane
       
        // first the near vector
        GLU.gluUnProject(winX, winY,  0, modelview, projection, viewport, pickingRayBuffer);       
        Vector3f nearVector = new Vector3f(pickingRayBuffer.get(0),pickingRayBuffer.get(1),pickingRayBuffer.get(2));
       
        pickingRayBuffer.rewind();
       
        // now the far vector
        GLU.gluUnProject(winX, winY,  1, modelview, projection, viewport, pickingRayBuffer);
        Vector3f farVector = new Vector3f(pickingRayBuffer.get(0),pickingRayBuffer.get(1),pickingRayBuffer.get(2));
This is my current code, now what do I add to find what part of the world my mouse is clicking on?

I need to be able to find the xz coordinates when a cirtain y value is inputted.
Ie, a method in a similar format to this:
public static Vector2f getCoordAtY(Ray ray, float y)...
Thanks in advance!