Photoshop comes with many built-in brushes that mimic natural media. Here are some basic techniques that you can use along with those brushes to create digital art that has the look and feel of traditional artwork
Sharpie Sketches - Horses by ColossalBeltloop . I once heard an artist say that she liked to draw in pen because there was no erasing, and it helped her learn to accept imperfections. That seems like a good (albeit somewhat painful ) idea!