first off google is a great tool use it. there are tons of tutorials to help you on the internet. secondly think about PMing Rex Raider. when it comes to photoshop he is god. and start off with basics, find a simple picture of a car off the internet and try testing basic stuff on it, try changing the paint colour, or add new rims. Maybe new tires, or drop the roof. Things like that. once you get more advanced and start doing chops for contests ETC. you need to pay more attention to little details, are all the joints blended thoroughly, does it seemr realistic, no glaring mistakes etc. Other than that you should get everything you need from rex or google. Hope it helps.
Try colouring one of your drawings too, they look good.
