You copy people you like. Copy their work like fuck - call it fan art tho'; don't steal it.
Then try and redraw their stuff from memory.
If you copy lots of different peoples' art then try and draw something similar. Then get stock photos and draw them in the styles you've learnt. Then try and redraw without looking. Then add stuff you think might look good. Stock photos are important.
AND THEN PRACTISE LIKE FUCK.
There. That's basically A-Level Art in a paragraph.
Oh - and start with the shadows if you're going realistic. And draw what you're drawing 100 times until you get the one you like most - obsess. This is something you love, isn't it? Then breathe it; draw it for hours, then keep going; try lots of different mediums just to see how you like it.
Also. Shadows arn't black unless you're in monochrome. LOOK AT THE COLOURS and REPLICATE THEM.
Finally. Credit people. It's important to be polite. It's much better than being sued