top-view shooters are without a doubt the easiest (graffix wise) games to make, you can get by without any animations, and worlds are generally just repeating textures with things placed on top.
pretty much the reason they're my game of choice to make to flesh out an idea.
but for the character, if your not to terribly great at drawing, i recommended using the bone moment, i sure you can draw simple shapes like circles and what not and with a little shading they can look pretty good.
textures for the ground you can just grab off the internet("free ground texture" Google)
objects and walls are probably your trickiest thing, but a birds eye view is kind-of an unusual way to look at the world, so you got a lot of room to mess up.
heres the beginning of a current TD-shooter i'm working on with the bone movement, to give you an idea of what i'm talking about.
wasd - move
(there is a lag in animation when switching to and from running, and just ignore the missing sound files) it is what it is.