right if you want to do it, get something like Photoshop Elements 7 - any older and its gets bad, it came out in 2010 so has all the tools you'll need and looks nice. Its dirt cheap on ebay, i got it for £25 a while ago because i didn't want to pay top wack for tools id never use.
Then get on youtube find out about these tools:
Select
Healing tool
Polygon Lasso tool
Brush
Eye dropper tool
Eraser - when youve got layers is an amazing tool haha
Basics in layering -
possibly the most important
Dodge / burn tools
Keyboard shortcuts - saves so much time when you get to grips with stuff
Don't start getting into masking and all that rubbish, basics is best. I've used nothing more than them tools for that second edit.
Once youve got the basics this is possibly the holy grail for photoshoping cars, ive learnt the hard way at college but this guy is amazing.
http://www.digimods.co.uk/tutorials/index.htm
and his work is here as well:
http://www.digimods.co.uk/index.htm
Hope this helps everyone starting PS haha