Well, what your asking for is more of something you'd find in a book, or in a class. Photoshop is way more than a MS Paint style program. That's why it's not free.... legally.
The things you're asking for are basically done using 'layers'. By default I the 'layer' layout should be on the right hand side, you can create up to 99 layers in a PSD file. Only PSD file can hold the layers, JPG etc will have to be 'flattened' to be saved.
For color, make a new layer and color on the car where you want the clore to show, on your 'layer' layout there's a pulldown menu with 'normal' as default, change it to different settings to see what happens. Also play with the 'opacity' slidder next to that pull down menu. Usually to change the color of a car it's atleast 3-4 layers, to ave it look right.
For putting different wheels on, find some pictures of ones you like and bring them into your car image. This will create a layer which you can alter to match your car. Like I said, it can't be taught over a forum board, but those should point you in the right direction. Do a Search for Photoshop Tutorials on Yahoo, I'm sure you'll find a bunch.
'87 GT Fastback T-top