Learn the basic tools and materials available. Art encompasses such an array of media and techniques, so understanding everything about every tool and material is not practical. A beginner's guide is an excellent way to get an overview of the art world. You can find many beginning art books at a book or hobby store, as well as excellent online sources see resources.
Some artists enjoy using a variety of media, while others prefer a specialty, such as sketching, painting or working with clay. The medium an artist chooses may be something she can do well, or she may simply prefer the medium. An introduction to art and its various media can be found in beginning art books or even online at one of the many websites devoted to learning art.
Regardless of the medium, sketching is one of the basic skills most artists use. Some are better than others, but at least a little basic sketching ability is needed to pursue most forms of visual artistic expression. There are several websites that specialize in teaching beginners how to sketch, such as drawingCoach.com.
A lot of artists begin by sketching, then move on to experiment with various painting methods. Get a beginning book on oil painting or acrylic painting techniques from your local hobby shop. You can also take an online introductory course. The best way to learn another medium is to just jump right in and paint something.