Brushing is difficult to teach - For example you can teach someone all the technequess of being able to to draw - how to judge proportion and shape, how to hold the pencil, how to get all the different effect etc etc but it still doesn't mean that the person could then draw a portrait. Brushing is about finding your own technique and practice more practice and some more practice. The way I found was try tooning a car but try to make it as close to the original as possible as it gave me the idea of how much detail to put into the paint work and how to achieve the lighting effects of cars in different backgrounds. This is all to get used to brushing and develop your own technique, this might not work for you as everyone is different.
General tips though -
1. Start small - don't try to instantly go for a full car, do some slight modifications to bumpers for example. Then work up to brushing more and more
2. Use a reference for paint and reflections and where possible a similar angle car or even a similar car - Take the S13 in my sig, the reference car is an S15 just facing the other direction.
3. Post a WIP and don't be afraid of negative comments because you will get helpful ones too. For a while I was too worried about posting up wips as they were rubbish but with people pointing out the flaws in the chop you get to improve them and fix them before people notice next time.
All I can think of at the moment but if I think of anymore I will post it.
