Determine what type of wood your banister is, as knowing this is important when it comes time to match the color of a stain. If you're familiar with wood, you can probably figure this out yourself. If not, ask someone who would know to look at your banister for you.
Decide on the overall look you want to give your banister. Even if you're not an experienced home decorator, it's likely that you have an idea about how you want the area to look. At this point, you don't need to know an exact shade, but it helps to know if you want the area dark or light.
Visit a home supply store that features a good selection of different types of stains. Browse through the list of available stains and look at the sample selections for each. At many stores, you can see samples of how each stain looks on commonly used wood, such as oak and pine.
Speak with a store employee who's well versed in stains and their application. She should be able to explain which stains work well with which kinds of wood after she's heard a brief synopsis of the look you're trying to achieve.
Browse through the different styles of stain to choose one that's right for your specific application. If the banister is outdoors, for example, you'll require a weather-resistant stain. Likewise, you can find stains with both glossy and matte finishes and styles that are completely transparent, semi-transparent and more solid in nature. If you want the grain of the wood to show through, for example, select a stain that's on the transparent side.