Photoshop expert advice, please

[Phimosis]

Hi. I have Photoshop CS3 but don't know how to use it like a pro.

When I try things like changing the color of wheels, I can never get it to threshhold the wheel and end up changing the color of fender and stuff too. What's your tick with this?

When I see Photoshopped wheels, sometimes they are shiny like they have a reflection map from a ray tracing program. how do you achieve this effect?

Sometimes I see different wheels put in a car , but the shot will be like 1/4 front or something and chopping the wheels from any other angle will give you oval shapped wheels that don't match the shape of the one's your'e replacing. So how do you stick aftermarket wheels under your car?

I see people take a picture and "lower" the car. I understand you cut and paste the wheel higher up in the fender well, but then what do you with the the background to make it look natural, instead of white cutout spots?

Lastly, if you have other cool techniques that you use all the time that I just would never ask about because I don't know enough about photoshop, please post them!

Thanks in adance,

-Phim