Start with 8bit images. pixel those for awhile then slowly start to move up to 16 bit, 32 bit, then keep going higher in resolution as you feel comfortable with each one, learning how to read colors and control how they work together. You'll get better as you go along. Pixeling isn't something that can really be "taught" or at least not in any way I know of.  It's drawing with blocks. You can't "teach" someone to draw exactly. Drawing is something gained from muscle memory. Same thing with pixel art really, memory in how blocks work together using their color schemes to form a picture.