If you buy the polyester batting, you don’t have to worry about it shrinking. These come in different thickness so you can get really thin or really thick batting. (for a beginner, I wouldn’t use the really thick stuff because it’s pretty hard to handle under the sewing machine) If you get the 100% cotton, like warm and natural, it’s usually prewashed but not preshrinked. It may shrink some after washing but it’s not much. (I think they say it can shrink 3%) If it does shrink, it may get some puckering in the seams. If you want to use the 100% cotton, but want to take care of the shrinking ahead of time, you can prewash it again before attaching it to your quilt. I would make sure to read the directions on the package if you do want to pretreat it.