It is called a footstall (Cause you stall it on your foot.
Me being a freestyle beginner myself, this is the one trick I can do very well. Of course, practice will make you better the more you do it, but here are some tips to get you started.
1) It is easiest to start out barefoot, so you have a better feel for the ball. What you want to try to do when the ball is on your foot is pin it back against your shin by pointing your toes upward. If you do this well enough, the ball will not be able to roll around, and possibly off your foot. If you feel it is about to, continue pointing your toes back, and move your foot the opposite way to balance.
2) Take the rollup very softly and easily. Start with your foot on top of the ball, and roll it back with your sole. Now, watch the ball. When you see that your toes are pretty much under the center of the ball, now you point your toes and try to pin it to your shin.