I use a Canon EOS Rebel T3i (600D) with the kit lens. I have experimented a lot with different settings and modes, but I feel like I have wasted many good shots that could have looked amazing, if only they were a little bit sharper.
I use auto-focus on most of my pictures and make sure to choose the right focus point, but all of my photos are still a little bit out of focus. When I zoom in, I can see that the pixels are not very crisp or well-aligned.
I am not sure if this issue is caused by my equipment or skills.
Answer
Technique is typically at fault with "fuzzy" images 99% of the time with someone new to interchangeable lens cameras (ILC) with only a low-cost kit lens.
The lens is not the problem. Low end kit lenses are limited, and they are cheap, and there are much nicer lenses around, but how you use one is more likely to be the fault than what glass is in the lens. People will often blame the lens because they assume basic mastery of technique, or because that's simply easier than questioning whether it might be a lack of knowledge/expertise, and because there are so many online discussions where people denigrate the lowly kit lens. But kit lenses are surprisingly good—especially for the cost—if you know how to use them. All you have to do is look at any Kit Lens challenge to see this.
Here are a couple of issues you need to resolve, as any or all of them could be involved in your images.
Handholding Technique
Nine times out of ten when I see someone shooting with an ILC, they're not holding it correctly. If your left hand is cupped around the left side of the lens, pinkie closest to your subject, you're doing it wrong. You want your left hand cupped under the lens/body, palm up, with your pinkie closer to you. You can still operate any lens rings with your thumb and forefinger in this position. And now your left hand is braced to support all the camera/lens weight, rather than having the camera torquing against your right hand. This is a far more stable hold that allows you to use slower shutter speeds.
Aperture Extremes
Aperture is a balancing act. On the one hand, the wider the lens is open, the more light you get, and the lower your ISO setting and the faster your shutter speed can be. However, any lens used wide open is at its weakest point. Most lenses perform markedly better stopped down 1-2 stops from wide open (the EF 50mm f/1.8 II is especially guilty of this). Chromatic aberration, vignetting, and softness can all be improved simply by not using the lens at its maximum aperture. A ton of your shots with your 18-55 are @55mm, f/5.6. Stop it down to f/8, and you'll see a bit more sharpness. And using too small an aperture—especially with today's pixel densities—can see diffraction making things softer, too. So, probably not good to use apertures smaller than f/16 unless you've got a good reason to.
Also, not shooting wide open gives you more depth of field, which in turn gives you more leeway with the autofocus accuracy. A 50mm f/1.8 II at some subject distances, if wide open, yields a DoF that can be measured in millimeters. Any slight movement of the subject or camera could throw the focus off. Trading off some background blur for better focus is often worth it.
Misfocus
This isn't actually as common a cause of fuzziness as most people assume; the first assumption most beginners make whenever something's fuzzy is that it's a focus issue. But when you let the camera's autofocus system take over, the problem is that the camera isn't smart enough to know what the subject of the image is, and direct the focus there. Learning the different autofocus modes (like face detection and eye-AF) and when to use them, how to select autofocus points or zones, how to AF track, and how to half-press and recompose are your weapons, here.
If it's an issue of misfocus, check the rest of the frame to see if anything else was in focus. With your wave shots, you'll notice that sections of the waves are crisp and in perfect focus, while other parts outside of your depth of field are not. If you'd stopped down to a smaller aperture (say, f/11), more of the wave would have been in focus.
Also, don't expect miracles in low light. Cameras need more light to "see" by than your eyes do. It's normal for autofocus systems to hunt in dark conditions. Your flash can send out more light for focus assisting, or you can use liveview and 10x magnification (if your camera's on a tripod) with manual focus when the autofocus fails you. Pay attention to the autofocus confirmation "green dot" in the viewfinder.
Minimum Focus Distance
If you came from the P&S/smartphone camera world, you're probably unaware that most lenses can only focus so close (this is why macro lenses exist). Small sensors mean equally smaller lenses, with very short focal lengths which equates to very large DoF, and very good near-focusing capabilities. When you jump up to an APS-C sensor, and your lenses get commensurately bigger, so does this minimum focus distance. The EF-S 18-55 cannot focus on anything closer than the 25cm. Your fuzzy close-up portrait demonstrates this. You'd need a macro lens, or the poor-man's macro methods (close-up filter, extension tubes, reversed lens) to shoot closer.
Shutter Speed Too Slow
Shutter speed can affect you more if you're using a longer lens, but even with IS, there's still a lower limit, and this assumes that you have good handholding technique to begin with. If you're shooting single-handed, if you don't know how to brace your feet or time your breathing, you'll need a higher shutter speed. 1/30s is a typical threshhold, and there's a rule of thumb about 1/focal_length or faster. Some folks would multiply that by 2, or throw in the crop factor as well. @55mm, that would mean using a shutter speed around 1/100s or faster. And that's with a stationary subject. With a moving subject, to "freeze" the motion and avoid blur, you may need an even higher shutter speed, and how high depends on how quickly your subject is moving.
Consider using physical stabilization as well for very slow shutter speeds: a tripod, monopod, or beanbag can make a big difference. Also, for macro shooting, everything is magnified--camera shake or subject motion included. 1/focal_length might not cut it the closer you get.
Postprocessing & Pixel Peeping
Don't pixel peep to judge sharpness, unless you're a lens tester :). Look at the image overall. Asking for ideal razor-sharp perfection on every pixel in any image is a big big ask. Very few lenses are up to that task, and certainly not a kit lens used wide open with bad technique.
Also be aware that P&S cameras, smartphone apps, and most photographers enhance the sharpness/contrast/saturation of an image through processing. Don't expect straight-out-of-camera JPEGS—unless you've asked the camera to add these things—to look as sharp/contrasty/saturated as images around the web that were shot RAW and post-processed with care and skill.
See also: Why am I having trouble getting sharp results with my new dSLR...?
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