Well the number one most important advice has already been done soo I\'ll give you number 2

When you go out start looking at things, the way the light falls on object, on cloths. If you want to paint somthing black try to see how black t-shirts look in daylight what color are the lightest parts and such. This is kinda hard to do, not only because you don;t always think aobut it ( or never in my case

) or because it is hard to see the \'different\' colors on an object.
And practise is also key, as soon as you\'ll practice a lot you\'ll see some improvments. I think for each technique you want to master you have to practice untill you really have the hang of it, or at least untill you get honorable results, then there is a good chance it\'ll stay. I found that if I only practice somthing for a week and then stop for a month, all that I learned in that week is gone.
And show us some of your work !!
