You probably already have the 18-55mm f/3.5-5.6 kit lens. It should be enough for most shooting conditions but admittedly, it's not enough for some rare moments.
If you find yourself backing up more often because you want to get more then get a wide angle lens, wider than 18mm. If you constantly yearn to get closer to your subject but can't, get a telephoto lens, longer than 55mm. If you are hoping to get more out of relatively low light or are wanting to blur backgrounds more easily when doing portraits, get a faster lens with an f/number lower than f/3.5 like the 50mm f/1.8. If you want to make small objects seem big then get a macro lens. It's all about what you want to do that your existing lens or lenses keep holding you back on.
Here's how to look at lenses properly.
Now you know!