Visible? if so, what to display? what functionality should it have? etc.

check out the last few pages of this tutorial: http://computingteacher.edublogs.org/files/2013/02/GreenfootScores-vvyjld.pdf

thanks any more help on how to make a simple game?

if you dont know what you're doing take the tutorials: http://www.greenfoot.org/doc

they havent helped at all though

They helped me; just look at them and when you are done develop your knowledge further by looking on the forum, source code of other games, et cetera.