You'll want something around 9" x 12" so that it's big enough to sketch on, but small enough to fit in a bag fairly easily. It's up to you on the quality you want. If you're really into drawing, might want to get a nicer pad with the rings, if you're unsure, by all means, purchase a little cheaper notebook. If you are taking Drawing second semester, you can use this same one.
Please note that the ones that are glued together tend to fall apart before the semester is over, though.
This is an example of a nice quality sketchbook.