Though there is much to learn when it comes to making images on ground, nothing helps reinforce the process of gaining expertise then keeping a sketchpad. The choice of type is personal. Over the years, I've gone from a wire bound large scale sketchpad, at about 14 x 17 inches, to a more hand held scale that measures about 5.5 x 8 inches. The paper in the sketchbook is important too. Some pads are made for multi media, others exclusively for watercolor and some only tolerate dry media like charcoal or pencil. You'll have to decide. I prefer multimedia, but keep a watercolor sketch pad too (You can keep more than one going at a time…). The rationale for the choice of size and ground is simple, easier to carry around, extract at a moment's notice and get down to drawing with whatever media you prefer. The brand of pad I prefer is Handbook, Pentalic and Strathmore and Handbook all make excellent watercolor sketch pads. Moleskine is expensive. There is also a German brand called Papier & Kunst that has a excellent product.
What you draw with is important too. In this regard, I favor Kuretaki brush pens. I chuck the Sumi ink it comes with in favor of Platinum Carbon ink. It is permanent and waterproof when dry. It is best used with synthetic haired brush pens. Do not use with a Weasel hair or Sable haired brush pen, it'll ruin them.