practise

The first two items you need are paper and paint. Much of the final outcome while painting depends on the quality and texture of paper. The most common ones are cold pressed, hot pressed and rough paper, but it is the cold pressed paper which is the most popular among artists due to its texture and versatility. 
But to start practising use a good grade student's paper.

There are hundreds of tube and cake watercolor paints available to the artist, and this might be confusing for a beginner. Many colors are made up of more than one pigment, and there are pigments that are transparent or opaque. Some are staining, some are not and can be lifted off even when dry. Be patient, soon you will want to experiment with different paints and textures.

These are the basic colors I usually start off with:

Ultramarine Blue 
Phthalo Blue 
Burnt Sienna 
Raw Sienna
Permanent Alizarin Crimson 
Indian Yellow 
Aureolin

Now it is time to practise! To create your first 'Color Symphony' start with the Wet in Wet method. This is an exciting exercise to observe the free flowing possibilities of watercolor. Practice often, it's fun.

Wet in wet is simply the process of applying pigment to wet paper. The results vary from soft undefined shapes to slightly blurred designs, depending on how wet the paper is.

Simply wet the paper with a large brush and paint. If you apply soft colors it will create subtle background regions for your painting. But be courageous! Apply strong colors and see how they blend, flow and move!
Let your first wet on wet painting dry thourouly!!! This is very important! Now you can add more paint!