To create great paintings, you need good paper. Even when practicing you need to use the good watercolor paper. Cheap paper will give you bad results and it will not help you improve your skills. The cheap paper will buckle the more water you use and it will not absorb the pigment the same.
I suggest using a minimum of 140 lb. archival, acid free, 100% cotton rag watercolor paper. My favorite paper to paint on is 300 lb. cold pressed watercolor paper. The weight of paper is the weight of 500 sheets of the full sheet sized paper. A full sheet of watercolor paper is 22" x 30". Basically, it boils down to the thickness, the thicker the paper the heavier it will be. The way to go is 140 lb. or 300 lb. watercolor paper.
Next is the texture of the paper. Cold Pressed, Hot Pressed and Rough are the three basic textures available for watercolor paper. Cold Pressed has a slight textured finish. Hot Pressed has a smooth finish, think heated like ironing out the wrinkles.. Rough has a rough textured finish. There are a variety of brands of watercolor paper. In most art stores, the paper is sold in sheets. Each full sheet will have a watermark sometimes in one corner, sometimes two corners and sometimes in the other corners you will find an embossed logo of the company who made the paper. If you hold the paper up to the light (as shown in photo 1) you will see a watermark in the corner.
With the different brands of papers, you will also notice slightly different textures of cold pressed and slightly different shades of color. Some paper comes in natural white and some in bright white. There is no standard to the natural or bright white, photo 2 shows the variation of natural and bright whites and also the textures.
Watercolor paper is also sold in blocks (photo 4). A block is like a pad of paper, but instead of having just one rubbery end, it extends all the way around the paper with just a little non-glued section where you can slip a palette knife in to separate the sheets of paper. Unless you have a rather dry style of painting in watercolor, I don't suggest painting on the block, you run the risk of wetting the sheets below over and over. You can remove one sheet at a time.
Whether you are using a block or full sheets of 140 lb. watercolor paper, torn down to 1/4 sheets (11x15"). Mount your paper on a stiff board (shown in photo 3) such as double thick illustration board, the backer board to an old block, heavy weight cardboard a tip for using these types of board is to run a few rows of masking tape around the edges of the board to help it last longer. A piece of Fredrick's canvas board works great too. Then using regular old masking tape, tape all four edges of your watercolor paper to your board. No need to buy fancy artist tape or stretch your paper. Stretching paper ... forget it!
With your paper taped down on your board you are ready to paint. Currently in my classes, we are using Saunders Waterford 140 lb. archival, acid free 100% cotton rag full sheets trimmed down to 1/4 sheets (11x15"). Other brands I have tried and would recommend are Arches, Fabriano, Lanaquarelle, Waterford, Winsor & Newton, Kilimanjaro. Try different brands, different textures and you will find one that suits you well.