What are the things I need to learn about painting?

Hi, I’ve always been interested in art and learning new ways of how to create art. Ive taken art in comp/high school in hope of learning new things, but to my surprise i haven’t learned a single way of going about with things and ideas.

I am now in college, i’ve taken up art again, and to my surprise i’ve only learned the surface part of art. Not even painting, shading, difference in darkness and lights techniques etc… I only see them from other artist and i usually just sit and wonder, ‘How did do it?’

My main area is sketching and observing but as an As level student, apparently sketches isnt enough to be a final piece to hand in as for our last work in our theme. This leaves me hanging not knowing how to go about a certain theme, and i’ve been working on painting but it takes me a lot of effort and time to do a painting… I finished one painting detailed in a week, 6 hours a day. And i wanna learn new ways. When i look at artist they focus either on certain parts of an object and I dont know if thats acceptable in As level as we have this type of ‘Developement ideas’ where it seemed to be very detailed and you have to put in a canvas more than one thing… but im confuse as they never explain anything or teach us anything. Lol.

I just wanna learn haha. Youtube seems to be my only hope really. Im trying to figure out what to do with my final for our theme of ‘Trees’ I was thinking of doing a family tree, in a huge tree that i’ve taken a picture of and play around with some of my primary sources to make a compisition but I dont know how to go about it and all…. And now ive seen some artist work in youtube http://funadvice.com/r/152aaf08i26 Im inspired by that artist and I wanna do something similar to her, but I dont know how to put it down into as a developement ideas or if its acceptable.

Any help from anyone whose taken art in educations?