Welcome!
This is just my two cents. Your mileage may vary.
When I first started (and still to this day, in fact), I focused on gameplay first. A good game is good, even with poor art.
Case in point:
viewtopic.php?f=77&t=74519
Besides, if you make a good game, it will be easy to recruit some help on art. Don't expect help without either something to show, or some money to invest, as artists know to be wary of the number of unfinished projects, and are hesitant to commit (quite understandably).
And yes, of course, check the links Amy provided to get better at art.
I just wanted to present the point that art alone is not what makes a game great. Personally, I tend to set up demos STILL using nothing but rectangles for my characters. Once they play in a compelling way, then I either recruit an artist, or the art slowly transforms (from rectangle to blob, from blob to humanoid, to human, to a character, to a detailed character).
Edit: just realized the d/l link died for GREEN. whoops. I'll fix that soon.