Newbie Q: creating a billboard rectangle with texture and alpha transparency?

I can't believe how non-intuitive and difficult Blender is for
starters. I've spent the better part of an hour trying this, and have
failed. Bryce took me 5 minutes. Ok, maybe 8.

Can someone guide me? All I am interested in doin at this time  is

- place a large or infinite ground polygon, perhaps a terrain, but a
simple square will do

- set the sky top half of the background to a blue color, or even a
gradient that fades to white near the horizon (also known as Los
Angeles grey, or Phosphor yellow)

- place a distant light for the Sun up there in the Sky.

- a great sky with clouds and sunset would do even better. Can the
distant light be matched with the Sun in the sky so the shadows
correlate automatically? (like in Carrara)

- place a rectangle into the scene, standing upright on the floor

- map a texture on it, from an image file in Targa format, or tiff,
with 32-bit depth, alpha included for transparency mask

- render it and see 'through' the mask where it's not opaque.

- and make sure it's casting a shadow that shows the opaque mask only,
not the whole billboard rectangle.

I want to show some results of particle brush based foliage rendered
in Blender, coming from PD Particles. Perhaps put it into a tutorial -
http://www.thebest3d.com/pdp/tutorials - anyone can give me a pointer
to the above steps?


Thanks. I know it can be done, I just am flabergasted as to why I
can't figure it out on my own. I'm not seeing that challenge with
Cinema4D GO or other apps so far.