How to make chapattis round and of same thickness?
If you are having trouble rolling dough to a uniform thickness then you might consider putting training wheels on your rolling pin until you get more practiced.


http://www.amazon.com/Rolling-Hills-Pin-Rings/dp/B000I1ZXBC
(I don't know anything about this particular brand.)  

These rubber bands fit on your rolling pin and act as spacers so you can enforce a strict thickness upon your dough. They are, of course, useful for rolling out many things besides chapati.

To reiterate what @rumtscho and @bob said. Your dough should be of a uniform consistency if you want to roll it out in a uniform manner.

As far as a perfect circle goes, turn it often and don't roll too much in one direction. It just takes some practice.