the rain effect itself is simple in photoshop:
New Layer
Filter-> Render->Clouds
Filter->Noise->Add Noise (Monochromatic and Uniform)
Filter->Blur->Motion Blur (about 120 degrees and between 10 - 50 %)
Layer Mode: Soft Light/Overlay (make sure this layer is always on top!)
but what mainly makes the rain images are the little details like the spray and the droplets etc, if you are new to photoshop then try getting Stock images of these sort of things or even easier, search for brushes on places like Deviantart
For Reflections:
Select The outside of the car using the lasso tool (Polygonal is easiest, magnetic can be a pain sometimes depending on the shades and colours and normal lasso if you have a really steady hand with a mouse or a Graphics Pad)
Copy and paste the selection of the car into a new layer, now flip the car vertically, now you need to press Ctrl+T Move the car so it looks like it has just been flipped and is connecting by the tyres
Now warp the car until you think it looks right and all the tyres are touching the original cars corresponding tyres.
Now use the soft eraser (recommended on a fairly big scale between 100px and 300px) to rub out most of the roof of the reflection.
Now set the layer of the reflection to either Soft Light or Overlay
For the rebounding rain drops around the car:
Create a new layer
use the hard circular brush (set to 1px and in white) now click the brush icon next to the colour and adjustments box, then click on Toggle The Brush Panel (Highlighted Below)
Tick the box "Scattering" then click scattering, change the scatter percentage to 1000%
Now draw around the top of the car with this, you may notice it is a bit too bright, now either set the layer mode to overlay/soft light and play with the opacity or just play with the opacity so it looks grey and see through and like it blends in with the background
the rest you will pick up over time
but if you are struggling, brushes do help a lot and as long as you don't use the brushes for any financial gain or advertising you can use them as much as you like
I hope that helped a bit
EDIT: Oh i must remember to say this is for Photoshop CS5, i can not remember what the layouts of previous versions of Photoshop are like so sorry I can't help, but it shouldn't be too hard to find where all the bits and bobs are