Google "GIMP how to paint hair" and you will find over 9000 tutorials. Also look up for, and look up 2d painting techniques (eg for oil or acrylic) since the techniques are transferable.
Take a hard look at some hair you do like, look at heads made by artists who are good and you can see the way they block out the area, add shades and highlights etc. It won't tell you exactly what brush settings or what work flow, but try to emulate it in conjunction with tutorials.
Personally when I was younger I would just set brush dynamics to make the size reduce over distance, then draw some nice anime looking hair and throw on highlights. Real basic shit.