You do need to use specially prepared paper to do watercolours. It is usually sized to prevent the paint sinking into the paper. Later you may decide to experiment but in the early stages it is best to use the same brand of paper for every painting. Paper is available in different weights. The weight refers to the weight of one ream of the paper or in the metric system to the weight of one square metre. If you do not use a lot of water or if you are prepared to stretch the paper before use 140lb paper will be sufficient. If you plan to do wet-into-wet washes, do not wish to stretch the paper before use or do not want to have to carry several boards holding different sizes of stretched paper I recommend using 200lbs paper. My own preference is Bockingford 200 lbs.
Paper can be obtained in three varieties of surface, "Hot Pressed" (this is good for detailed work, "NOT" (this is an acronym for "Not Hot Pressed" and is good for general work) and "Rough" (this is used to produce textures, or to get the sparkle on areas depicting shimmering water).