It's a good start bud!! There are a few things i can help you with, but like i said its a good start!
Your skull seems to be to dark and have outlines. Kind of like a cartoon has the black outlines where it was drawn. You don't really want that as you go forward. Work lightly and blend you black, fade it lightly to the darkest points of the shadows. There is almost nothing straight on a skull that had a straight sharp line, everything is rounded and that's where the fades with create the shape and depth at the same time.
The blue flames look way better. You have to much stencil mixed in there. Do a loose free hand deaign and then find the curve of the stencil that fits the curve of the stencil size and just spray the tip or peak of the stencil. Right now It's the whole round part of the stencil and doesn't really flow like a real flame. You also can go over them with a sky blue color then a little white and that will bring them out to give depth. It takes a ton of practice to do flames really good and I'm not great at it but I can do them pretty good.
If I get time I will try a little how to on them the way I do them. You have ago of start because I can tell it is fire and I have seen some that I had no clue it was a fire attempt,

!!! Keep going, and you will get it, for less than a week you have made major progression from the total newbie stage!!
Josh