My first-time pictures of Ren, Stimpy, and Kirby were almost at the exact same stage as Zach's drawings, so I can relate.
Zach, The most important thing is gaining familiarity and confidence with your subjects, most everything else is just gravy that'll come naturally later. Look at lots of pictures of the characters you intend to draw. If they've been animated, it even helps to see them in motion and freeze-frame DVD's extensively.
Basically, your drawings are good. The lines are a tad wobbly, but the construction is relatively solid, and most of Billy's and Mandy's features are just about where they should be. I'd recommend just tightening up the composition.
Zim: "I thought the Tallest killed you."
Skoodge "Yeah, but I'm all right now."
Mad Jack the Pirate: "You were DEAD! How do you get better from being DEAD, you Scottish twit?!??"