One of the main reasons to own a nice SLR camera is the flexibility of changing lenses to suit your subject. There is an almost endless supply of lenses to fit every situation. The is leads to a frequent point of confusion for new photographers: Which lens should I buy? Well, it depends. What are you trying to photograph? What kind of camera will you buy next? And the big one: how much do you want to spend? I can't answer those questions for you, so let's go through some basic information to help you make your decision.
Remember, our rule about photography "rules?" It definitely applies to lens selection. Don't think of these guidelines as absolutes; guidelines are never absolutes.
If you want to do landscapes, you'll want a wide-angle lens (<40mm). I think everyone should own a "normal" lens (40-50mm) . Portraits usually look better when taken with a mild telephoto (85-135mm). Sports and wildlife often require super-telephoto lenses (>200mm). Then there are specialty lenses to consider: macro lenses for close-up work; tilt/shift lenses for perspective correction (architecture); and many other fun lenses. So, what are you trying to do?
Lenses can easily outlast your camera, especially if you're a new photographer. Most new folks don't jump into photography with an expensive camera. This means you probably bought a camera with a CMOS or "crop" sensor that is slightly smaller than the top of the line "full frame" sensor cameras. There is nothing wrong with these cameras. They give you every opportunity to make wonderful photographs, and you can use the complete inventory of your camera maker's lenses.
How do you know which kind of sensor your SLR uses? You can find that in your owner's manual or look it up online. Here is a generality: If you don't know, it's probably a CMOS sensor.
Because of the lens design characteristics, "full frame" cameras cannot properly use a lens designed for the crop sensor camera. A crop sensor lens will fit on the full frame camera, but the light it transmits and focuses on the sensor doesn't provide enough light to cover the entire sensor. You will get a photo, but at reduced quality. However, a full frame lens on a crop sensor camera causes no problems because the camera's sensor is completely exposed, and the extra transmitted light is ignored. So you need to consider what kind of camera you'll buy next. If you are sure you'll never buy a full frame camera, you can forget the distinction. But, if you may someday move to a full frame, I recommend you only buy lenses designed for that full frame camera, even while you own your crop sensor camera. I've gone through three different cameras in the last seven years, but my lenses are still here. All of them work with my Nikon D4 (full frame) even though my first digital camera was a D80 (crop sensor).
This leads to our third consideration: How much are you willing to spend? It should come as no surprise to you that higher quality lenses come at a higher price. The latest super telephoto from Nikon is an 800mm focal length at about $17,000, but a really good normal view lens, 50mm, is about $150. That's a pretty big difference, and you can find the rest of the lens prices spread between this range. It isn't just about focal lengths. High quality lenses have a better build quality; are weather sealed; and have a constant aperture instead of variable aperture.
I'll go deeper into many of these considerations over my next few blogs. You can see why there is no single answer for which lens you should buy. The decision is part of the fun.