What lens should I buy?

April 15, 2014
How to decide on which lens to buy
First, lets start with a few questions. 
Do you plan on shooting landscape (need a wide lens) or animals (need a zoom), or both? 
Do you want something that is pro quality, or just average? Thus changing your price point. 
Do you plan on shooting your camera on manual controls, meaning are you going to shoot NOT on "auto" ? 
So... to elaborate... 
To understand what type of lens you want, you want to dive into the photography basics. Do you know, or plan to learn, about how to shoot fully manual? I highly suggest it, as its not a complicated thing to understand. Once you understand it, you will want a better lens because you will understand how to use it to your advantage. 
For example, once you know and understand how to get a nice shallow depth of field (blurry background, clear subject) you will want a lens that can accomplish that. So, here's my 60 second photo lesson, and Im going to be super basic.  I apologize in advance for being extremely elementary,but this is how I teach photography to people. 
So three main elements make up the exposure of a shot. The shutter speed, the aperture, and the ISO.  Ignore ISO for now... And lets talk about shutter + aperture. 
First: SHUTTER SPEED.  Imagine a curtain on a stage that opens and closes from outside-in.  The curtain is closed at all times, but you can open it and close it again, giving the audience a quick view of the stage for a period of time. This is equivalent to the concept of shutter speed.  The Shutter speed is the amount of time that the "curtain" inside the camera body is open to let in light to record what it is looking at.  So in basic terms, when you have a lot of available light (ie: sunny day outdoor) that "curtain" does not need to remain open for very long.  In fact, a sunny day the shutter (or "curtain") may open and close in a matter of  1/500th or 1/1000th of a second - which is considered a fast shutter speed. Literally a "split second".  In a dark restaurant at night, your shutter speed would need to be slower, allowing that curtain to stay open longer. Such as 1/60th of a second.  On the camera these numbers will appear without the fraction. So if it reads your shutter speed is set on 60 - just remember to put 1/ before it and that is 1/60th of a second.  In addition to light, shutter also can affect blur of your subject. If it is too slow, and you move the camera or your subject moves then you will blur the image. This is motion blur. It is different from depth of field blur due to aperture, see next paragraph. 
Second:  APERTURE. This is where your debate on lenses comes in a little more.  The aperture is the size of the opening inside the lens. So, the same as shutter speed this also affects the amount of light that is let into the camera. Again, more light needed when in a dark situation and less light needed in a bright situation.  The aperture is measured in f/stops which relate to the mm size of the opening... long story short, the larger # is the smaller hole/opening (ie: f/22 which is less light) and the lower number is the larger hole/opening *(ie: f/2 which is more light). The secondary part to aperture, is the effect that it gives you in terms of depth of field, or blur between your subject and background. Such as a clear subject, like a face, with a blurry background, like the trees.  This is a useful tool when shooting a portrait. WHere as with a landscape, you want the opposite effect - where everything is clear.  For a shallow depth of field, you want a large aperture opening, such as f/1.8, f/2, or f/2.8.  For a deeper depth of field where most everything in the shot is clear, you want a higher aperture (small opening) such as f/11 or above (max typically is f/22).  See below links for visual descriptions of each shutter and aperture. 
Visual descriptions of shutter and aperture:
Now, just putting the two together is the basics of photography. Since you are investing in a good camera, you NEED to learn how to adjust your shutter and aperture to get the look in the camera that you want. Otherwise, if you buy this camera and just shoot on auto- you are wasting your time and money. There are great point and shoot cameras for that. A good SLR is not going to get you a better image on AUTO than a decent point and shoot. The "Auto" mode is going to give you pretty much the same results across the board.  
So, in short, to get the right aperture you must learn and understand what effect each will give you.  I found this tool (link below) years ago and continue to go back to it to show people how combining the shutter and aperture affect your shot.  Click below and scroll down just a tad to the section with the photo of that couple.  On the right of that you can change your shutter, aperture, and then click "shoot it".  This will give you the simulated result. Play with this for a few minutes, then continuing reading on below. 
You will see that if you "shoot" that couple on 1/8 of a second, they turn out blurry, becuase that woman is obviously moving in the shot. 
You will also notice that if you shoot on a fast shutter + small aperture (Ie: 1/500th at f/22) it will turn out black. This is because it was not letting in enough light for that scene. 
And lastly, you will notice that changing your aperture settings will give you a different view of the buildings in the background, blurry vs. clearer. 
Now that you have seen and understand the basics of shutter and aperture, you can now go back to thinking about lenses and what type of lens you might want.  This takes me way back to the top of this page where I explained the three questions to ask yourself: 
What do you want to shoot? What is your budget for this lens? Etc. 
I think the most important thing to consider in a lens is the minimum aperture. 95% of the time in shooting situations you won't need a higher aperture (more than f/22) because your light levels will not need to get any darker. But VERY often in shooting situations you need the exposure to get lighter. For example, when shooting a moving subject like a child indoors - you will not be able to shoot any slower of a shutter speed than 1/250th of a second or else the child will blur. But shooting at that speed will certainly make your images much darker.  Therefore you will need to make up for it by gaining light through your aperture.  This gets into the details of lens purchases. 
If you have a lens on your camera that has a minimum aperture of 3.5, that is going to be the maximum amount of light that you can use on that side of the equation. Meaning that you will have to rely on a slower shutter speed to get enough light (or higher ISO which I won't go into in this blog until a later time).  A lens that has more f/stops beyond 3.5 can be purchased, there are many that have a minimum of 2.8 and some that go as low as 1.2 - there are many factors that go into the price that determines this. However, there are a few great options without needing to spend a ton of money.
One key point to know is the difference between a lens that zooms, and a prime lens.  An average zoom lens is a 18-55mm with a minimum aperture of 3.5-4.  What exactly does that mean? It means that your focal range for zooming is between 18 and 55mm, which is the ability to zoom just across the room approximately. And the minimum aperture on the lens would be 3.5 when on 18mm and after zooming into 55mm your aperture drops to 4.  This is because the light has double the distance to travel through the lens before hitting the body of the camera.   However, in a prime lens, also called a fixed lens - there is just one focal length such as 35mm.  There is no ability to zoom. That is the downside, however there are many positives and when I explain it you will realize that not able to zoom isn't really the worst thing.  First of all, a prime lens is a much higher grade glass due to the fact that does not have to be made to change focal lengths. A lens that zooms has to compromise some of the quality of the lens glass in order to make that an option. Also, the zoom lens isn't that necessary when you are only going from 18mm to 55mm.  Honestly you can walk in closer to your subject in almost the same amount of time you can twist the lens to 55mm, since it is only zooming a few feet across the room. I call this "zoom with your feet".  
SO, what are you shooting??
Your choice on a lens needs to be decided upon based on what it can do. Do you plan on shooting all landscapes? If so, a shallow depth of field may not matter to you. Do you plan on shooting people? if so, you may want a prime lens with a 1.4 minimum aperture. 
How much do you want to spend??
Spending more money means that you will get a lens with better glass inside, and better options on aperture for a zoom lens.  It is not expensive to buy a prime lens of 50mm at f/1.4.  In fact can be purchased for between $200-400.  But if you want to get a lens with the capability of zooming, AND a wide aperture, you will pay a lot more. This is because it is much more difficult to make a zoom lens that will zoom, and stay at a fixed minimum aperture without losing light when it zooms.  
When looking more closely at the prices on those lens pages, you will see the differences in the lens prices. For example, at this link you will see a 18-55mm 3.5 for $119, and also a 17-35mm 2.8 for $1900.  There are big differences in those two lenses, including quality of the glass and the minimum aperture size.
This really just breaks the tip of the iceberg, but hopefully it will point you in the right direction. There are many more things to consider, including whether or camera is full frame, or has a cropped sensor and more. But this would be an extension from this point to consider researching.