I tried to do head texture in paint (Mapping in Gimp)

(I'm learning, please don't hate me)

Well, it isn't bad for a beginner. Have you mapped it onto a sphere or tried it in game?

I mapped it only in sphere, so i don´t know how to looks in game

nice, but try to keep the lines straight and not very ridged like here

Well, the sphere is the same as ingame, really. Can you show a screenshot?

Also, use the pentool, and make the lines thicker. around 6. It will look good that way.

It´s possible edit with some effects in GIMP?

With this, I recommend starting a whole new piece. But it is possible to edit this.

Use the pen tool, the smudge and blur tools, the smudging and blur will help with your shading. Plus, use many layers. it'll save you the time.

Because it is your first job, you are to be congratulated