Hi BamBam80. The above suggestions are good ones and I also like the Magic Lantern books since they cover virtually everything in the manual but present it in a way that is much easier to read and understand.
I don't think you said which DSLR you bought, but in almost certainly will have various methods of setting the autofocus. You might want to try each of the methods to see if one works better for you than another for the kinds of things you are shooting. I often use the one on my camera which focuses upon the very center only. Autofocus requires sharp horizontal or vertical contrast lines, and you may be able to place the center of the through-the-lens view right on a feature like that, press the shutter release half way to lock the focus, then re-compose the picture before pressing it down all the way to release the shutter.
You will also have a diopter adjustment probably above the viewfinder. Make sure that is adjusted so that all of the information you see within the viewfinder is sharp.
Finally, there is only one plane in the subject for which the focus is perfect. With increasing distance away from that plane, in front or in back of it, the focus starts to blur. It is acceptably sharp (depending upon how the image is viewed) for a width that depends primarily upon the aperture or f/number you select, the distance between the lens and the subject, and the focal length of the lens.
If you set the aperture to a large f/number (smallest hole size), say f/22 or f/32 even, that will give you the widest possible depth of acceptable focus, other factors being equal, and if you select a small f/number like f/4 or f/2.8 if your lens supports that, then the depth of acceptable focus will be narrower.
For a given focal length lens, the further away from your subject you are, the wider the range of acceptable focus, and the closer, the narrower. For a given distance from the subject, the higher the focal length, the narrower the apparent range of acceptable focus (because this is essentially the same as being closer to the subject.)
So I suggest you try shooting some experiments in Aperture Priority mode, usually something like Av on your mode dial, where you set the aperture at as high a number as you can without the shutter speed the camera selects falling below 1/60 second or 1/30 second if you have a shake reduction feature on your camera. Or if you have a tripod or some other way to make the camera motionless, that would also work. Obviously, if either the camera or the subject is moving significantly relative the the shutter speed, it will be impossible to make a sharp image no matter what you do.
If you do all of this and still find that your images are soft or blurry, it is possible that there could be a problem with your camera's autofocus system, and while that is very unlikely, it can happen and you would need to have it adjusted under your warranty.
Postscript - It occurred to me that I forgot to mention that most lenses are not at their best for sharpness either wide open or fully stopped down. So the recommendation of f/22 or f/32 would not be correct. Take the widest aperture available for your lens -- the smallest f/number -- and reduce the hole size by two stops. They run like this: 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32. So if you have an 18-55 zoom kit lens that has a minimum f/number of 5.6 at 55mm, I'd suggest using f/11 at 55mm or f/5.6 at 18 mm, or if you happen to have a 2.8 then you might try 5.6 or 8, again using some means to insure your camera is not moving.
The other thing that occurred to me to avoid any depth of field issues altogether would be to use perhaps an article of clothing with striking contrast lines, such as a high contrast plaid shirt for example, that you might hang up so that your subject will be essentially in a single plane in strong light, and then try blowing that image up to inspect for focus.
Last edited by chicagojohn; 07-22-2012 at 09:09 AM.
"'There's more to a picture than meets the eye; hey, hey; my my." - Neil Young