Understanding focal length
After getting to grips with the standard "kit" lens that comes with a new DSLR camera, many people ask, "What is the best lens for me to buy next?"
The answer is that lenses are not good or bad. There is no single best lens to buy. Lenses are designed to do different things to achieve different effects. The best lens for you is the one that can create the kind of picture you are trying to achieve.
To figure out what you need, it's necessary to understand focal length. Focal length is the measurement of how much of a scene your lens will capture. It is also how we name lenses: 24mm, 50mm, 85mm and so on.
Here is the important part:
The smaller the number, the wider the angle of view. The higher the number, the narrower the angle of view.
So if your goal is to capture a wide scene with your DSLR, like a landscape or a whole room, then you should use a lens that is 35mm, 28mm or lower. If your goal is to capture a scene more or less as the human eye sees it, you should use a 50mm lens. If you wish the angle of view to be narrower (or zoomed in) you should use a lens with a higher number such as 85mm, 105mm or 200mm, since this is ideal for photographing things that are far away.
Here are some examples: