Let me start by saying I'm a complete beginner. I can't work out if it's because I've accidentally dropped the lens before (it fell out of my bag onto concrete 
), or because I use it with a 7 year old 550D that has also been dropped and never cleaned, or whether it's just my settings, but most of my pictures are turning out horribly. I've been trying my hand at concert and candid photography. Sometimes the camera will only focus around the sides of the image and not in the center like I want it to. Sometimes subjects look ghostly(?) around the edges and not sharp at all. If dropping the lens has affected the focus/sharpness, is it easily repaired? Probably very expensive, I know, but it's still under warranty if that helps. And I currently live in Korea which seems to be good place for camera needs.
So much dropping here it kind of hurts to read.
None of the things you describe really sound like they are clearly the result of physical abuse. I am having difficulty envisioning the ghostly effect around the edges but it could just be out of focus bokeh which most people actually like.
Focus issues. Simplify and isolate to test. Press the button on the back of the camera that looks like a checkerboard. It is the autofocus point selection button. That gives you a few seconds to look through the viewfinder and pick the center autofocus point as the one to be active. You toggle through the selections by turning the main dial on top of the camera near the "take a picture button". Pick the center point as the only active point and you then put that point on top of your subject, telling the camera "THIS IS WHAT I WANT TO BE IN FOCUS, CAMERA". If you don't do that the camera leaves all the autofocus points active and the camera decides what should be the subject and it will make that thing in focus and it may not be the thing you intended.
Depth of field in focus has limits, and is controlled by your aperture (f/number). Not everything in the photo can always be in focus. Something will have to be out of focus in most cases. If you have not gotten familiar with the "exposure triangle" this will sound like gibberish. Google for and watch about 3 short video tutorials on the exposure triangle. One of the 3 things on the triangle you can manipulate to control how much light comes into the camera is aperture. That is the size of the opening in the lens. The lens can constrict the opening inside itself to restrict light or it can open itself up all the way to let more light in. A side effect of this, however, is the depth of field in your scene that will be in focus. Shooting wide open can give you a very shallow zone of things being in focus. If you've ever seen a photo of a person and their eyes are in focus but everything in front of or behind the eyes is blurry, that was done by opening the aperture all the way up, plus some other measures to enhance the effect. If you have ever seen a landscape shot where things at all distances are clear and in focus (flower in the foreground close up is in focus and a tree 50 feet away also sharp, and mountains mile away in the background sharp too) , that was the opposite aperture setting, i.e. It was shot with the lens opening pinched down to nearly a pinhole size, which makes the plane/zone of things in the scene being in focus very large. Your 70-200 lens is a beautiful tool with a very large aperture and that combined with a long focal length of 200 means it can create a much shallower depth of field than a kit zoom can. That is great when you are aware of how and why it happens and you are expecting it, but it can mess you up if you don't.
@Lottie wrote:Let me start by saying I'm a complete beginner. I can't work out if it's because I've accidentally dropped the lens before (it fell out of my bag onto concrete
