I paint mostly on watercolour paper and here you have two options to choose from:
Hot Press or Cold press paper.
Hot press paper is a very smooth paper and is often used specifically to paint a botanical painting. I have used hot press paper before to paint a normal painting, but I find working on this paper makes me feel quite restrained as it demands a more meticulous painting process. I prefer working on cold press, especially when I am painting a Plein air or an urban landscape painting.
I find I like cold press paper with a bit of a rougher surface as I like how this interacts with the paint. It does take a bit of getting used to because you have to integrate the paper texture with your painting. The paper texture adds to how someone looking at your work would experience it. I find a rougher texture on a page allows me to work in a more expressive and gestural way, but I would advise those starting out to experiment first with smaller pieces of paper (or a watercolour pad) before spending a bit more money on buying a sheet of watercolour paper.
The thing to keep in mind with watercolours is the more you work on a painting, the wetter the page becomes so you have to stretch the paper onto a surface first. I like to be able to move my painting around a bit (especially if I paint outside) so I often work on an A2 piece of masonite or chipboard. It is good to find something that won't bend once you have stretched the paper as once it dries it could pull at the surface it is stuck to. Marine ply could also be a surface to work on, but it's quite heavy to move around.
So, to stretch the paper onto a board I use a brown paper tape that has an adhesive substance on the back that is activated once it gets wet-much like the inside of an envelope. As I often work on quite a large format I fill a spray bottle up with water and spray the back of the tape to get it wet (as licking a long piece of tape is not a fun experience!) I also pre-cut the tape so that it extends to the ends of the board (see pictures below). I then stick a double layer of tape on each side of the paper and leave it somewhere in a warm spot to dry. Once the tape is dry I put an additional strip of masking tape on the inside of the brown tape which I remove once I am finished with the watercolour painting. I also cut the picture free of the brown tape by cutting on the line where the brown tape meets the paper so that i can use the board again and so that the picture itself does not sustain any damage.