I'd suggest starting with cotton since fleece is a little stretchy which can add a little complexity. If you don't want to start on an actual quilt, make sandwiches out of excess fabric...14 or 15 inches square, or even fat quarter size, then practice away. Using the small size makes it easier to learn, I think. Once those pieces are filled, you can trim them to make them into a tote bag or other item, so the fabric and work is not lost.