Jelly rolls are 2 1/2 inch strips that are the width of the fabric. A charm is a 5 by 5 square, and a layer cake is a 10 by 10 square. I buy them all, plus I buy yardage. It depends on what you want to make. Charm pack quilts or charms, whether you cut them yourself or not, are usually good beginner quilts to make. You can use the 2 1/2 inch strips for sashing also. The 10 by 10s can be cut into four 5 by 5s. The strips can be sewn together to make pot holders and table runners and place mats and quilts also, either uncut or cut.
I would suggest you start slow and make an easy quilt first. There's lots of them that have free patterns on the internet.