I'd definitely suggest taking advantage of the Tutorials section on this site.  Of course there are many styles and approaches, but I've found much of pixel art is about the form or shape of the object in 3D.  As opposed to a simple, colored in drawing.  Similar to painting.  I'd suggest work on the simplest, largest shapes only.  Not only does it force you to think about what the most essential elements really are, but also makes it easier to know when it's right before you move on to greater detail.  Like this coin, just make a plain flat disc first. without the contours.  As well, a good approach is to try to reduce the amount of colors being used to a minimum.  This coin for example could be done using only the lightest 3 colors.  Hope this helps!