Hey, a couple thoughts:
The face is cool, I too like the cartoony style.
My advice is do all the designing on paper. Draw it out first, a hundred times if need be! That’ll give you a really good idea of how he looks from every angle. Modelling and character design are two different animals, so-to-speak, so don’t try to tackle them both at once! (Sorry if I’m using too many metaphors, your English is fine though, so I hope you’ll understand them.) Most character design is done by concept artists using traditional media or digital painting tools before being handed over to computer modellers, texture artists, etc.
What I’ve been told about character design is to focus on the silhouette first, round shapes are the most “friendly” and spiky shapes are the most “hostile” to a first impression at least. You ever wondered why bad guys have big shoulder pauldrons, and frilly capes, and massive helmets? It’s to make their silhouette more hostile looking. (I learned this at a character design workshop at University of Central Florida, so it should be pretty solid information!)
The skull man has potential, remember this is 3D! So be careful with anything flat or straight, it’ll probably look better and more alive if you use some curves!
I started Blender when I was about your age. That was a few years ago now, and I honestly sucked then. But I HAVE gotten a lot better because I stuck with it. Using this forum is a good idea, and make sure to spend some quality time on CGCookie.com or Blenderguru.com. CGCookie is probably the first place to go for your kinda thing. Be sure to ask any questions you may have! That’s the best way to prevent making mistakes, learn from other people’s mistakes.