ok im new to autobody and working on my car and i had a ding on the round part of the fender and i just cant get it to be perfectly round...hammered it out just enough and put filler on it but i just keep over sanding it...is there a trick to doing this??

 

there really is no "trick" to it, but there are many techniques you could try. In your case I like to use some type of block with a bit of flex to it and always try to sand accros two diffrent "planes" of the fender never in straight lines. You kinda just have to get a feel for it. It might also help if you prime the area with a polyester filler primer and block it again.