Lesson 1: Part Two - Choosing the watercolour paper...

Oranges,  watercolour on 300gsm rough paper

So what paper should you use? 

A good watercolour paper should be acid free, and contain at least 50 % cotton. Also the weight is important - a 300 gsm (140 lb) is a common choice, and will suite most of your work. 

A lighter one (200 gsm/ 90 lb) can be used for sketching, but tends to buckle. 

A heavier paper  (640 gsm / 300  lb) is fantastic, and you should try it, but it's really not necessary for the works of  a beginner or most more advanced painters either. Also the heavier papers tend to absorb more of the pigment, making corrections difficult!

Now you have the choice between the different finishes of the paper!!

 Hot pressed is the smoothest surface, it's also great if you want to do an illustration or a more detailed work. A cold pressed paper is a bit less smooth, and the rough paper, as the name tells, has a rough surface, which will add some structure to your painting and will not allow too many finedetails. 

Also the finish can contribute to your painting, as you see in the detail above: 

the rough paper was the ideal choice for the surface of the orange peel!

I recommend trying different papers - and also different brands. 

You can find great paper in blocks, where the paper is glued on the sides and easy to use, especially when travelling or painting outdoors. 

Personally  I use single sheets. They are larger, can be cut into the size you want, and also it is more priceworthy when you paint a lot. But remember that the sheets have to be taped to a board before painting!
You can read more for example here or here.