Nice job! The first thing I notice that could use some work is the line roundness and line intensity.
I notice in the full version your lines are a bit fuzzy, though it's not clear (and not a problem) in the small version for your pet overlay if you do plan to do larger pieces you may find that to be irritating. The other thing is the way your lines end in a rounded way, there is no depth or dynamics to them unlike your signature which it would seem you got the hang of both of the things I'm talking about (Being roundness and intensity)
Working on those two things will make your lines crisper, cleaner and much more appealing.
Other than that it's a very nice first try. The colors are adorable and for your first use on a tablet it's pretty damn good. My first was an atrocity!
^^ I didn't draw my forumset art, actually. did. The larger version is like the working size, I usually downsize everything I do by about 50% for the final version unless I'm working with vector. I think getting better with the tablet will fox my lineart problem, I turned the pressure sensitivity off because I'm just not quite good enough with it yet.