Many people outside of the art world think of watercolors as using a clumsy brush with those crumbly cakes of watercolor paint that you used to use in elementary school. Once you start investigating watercolors, though, you find that there are several tools you can use to create dreamy, colorful paintings. Chief among these are pencils, pens (such as tubes), and then the traditional brush. If you are considering using watercolors, your desire for specificity and control, as well as the longevity of your supplies, will drive your decision on which tool to use.
If you want to ensure that your supplies last a long time, pencils may be best for you if you are only dabbling in watercolor. While pencils are consumable -- they disappear as you use them -- they don't dry out. With a brush or pen, the watercolor ink you use can dry up, making the brush a mess if you haven't cleaned it properly, and requiring you to buy new paint palettes for use with brushes. If you're not going to do watercolors that often, pencils are your best choice.
Control Over the Picture
Watercolors really aren't supposed to be as exact as pen-and-ink drawings, but it's a lot of fun to produce something that looks like one medium while using another that's notorious for not being as precise. If you'd like to do that with watercolors, pencils and pens are best, with pencils edging out pens. Pencils let you sketch your painting first and also let you gradually add more color to a particular spot. With traditional brushes, you don't have nearly that much control over color saturation.
Control Over Your Hand and Arm
If you really want to develop your ability to use watercolors, a traditional brush is what you want. In order to make the picture more exact or to have better control over where the brush and paint go, you need to have excellent control over your arm and hand, and using a brush is the best way to develop that. Using pens and pencils does help, of course, but not to the extent that practicing with a brush does. With brushes, the bristles can suddenly move in ways you don't want them to. To reduce that, you need excellent control over how the brush lands on and moves over the paper.
All of the tools for watercolors let you create some nice paintings, and there's really no one right way to do things. Start experimenting and taking classes from local studios, such as Colorest Inc-Art Supplies, to figure out which mediums are right for you.