There's a vast range of sketchbooks on the market. I suggest starting with something simple and inexpensive.
Two sketchbooks are enough to begin with:
"Moleskine" is a popular high quality brand but they're expensive. They are hardbound which enables you to draw across facing pages. Some are lined, others can take watercolour, and there's a wide variety of sizes.
Strathmore is a good all-purpose brand and they do a wide range of sizes, papers and binding. Wire-binding is useful because the pages can lie open flat. Binding can be along the short edge (TOP binding) or along the long edge (SIDE binding). Use whichever you prefer. I prefer side binding as the pages are then in a portrait format (long rather than wide) which better suits my day to day work.
Above: Strathmore sketchbooks, wirebound on the long side, 30 sheets in each; 9 x 12 ins and 11 x 14 ins. Microperforated so sheets can be easily removed. The paper is a good weight and surface for drawing, slight tooth, and although it says "dry media" it can take some wetting. Available in the US from Dick Blick and many other suppliers. In the UK try Amazon.co.uk or Castlehill Crafts.