Suggestion:
If you want decent sprites, try pixelartmaker.com - It's great for beginners (Easy interface, large spriting space, adjustable pixel save size), but you have to have a computer. Likewise with Piskel. I agree with Gigavoir though - don't use GIMP right off the bat.
For my sprites, (which I think are decent), I use pixelartmaker.com for weapons and only switch to Piskel for gifs/redoing a sprite. Gain some exp with PAM, then move to Piskel, and then use GIMP (If you still want to, I only use it to assemble large sprites).
For example, the Kraken sprite I did recently was created in two halves with pixelartmaker.com, then glued together in GIMP, and then finally transferred to Piskel to animate the tentacles.