You can cut a square of cardboard to trace around. If you cut several squares at once, you've got spares for when the first one gets dog-eared. I've used pieces of cereal boxes, cracker boxes, Kleenex boxes, and so forth.
Or you could look at "quilter's rulers" because they are usually made of...
I suggest you start by making a small quilt, maybe for a doll. (Or a baby quilt, or a potholder, or a placemat for the table.)
No matter how much you read and ask questions, there is a lot you learn just by doing. Personally, I prefer to make my first mistakes on a small project :D
Stitching...