Just to clarify...
There are three major components that go into auto focusing a modern camera: the camera itself, the lens on it, and the user operating the two of them. To be honest, it sounds as if you don't have a thorough understanding of auto focus and using it with your particular camera and lens, so I'm leaning toward "user error", rather than any problem with the gear. Not to worry, we've all been there at one time or another with our cameras! Maybe this will help...
First of all.... There is no "auto focus vs manual focus setting" on the camera itself. None at all. The only place you can actually turn AF on and off is at that switch on the lefthand side of the lens. If you want to use auto focus, this switch must be set to "On". And If you want to focus manually, this switch, on this particular lens, must be set to "Off".
On this type of lens, if you operate manual focus by turning the focusing ring on the lens.... effectively overriding autofocus while it's trying to operate.... you will damage the autofocus mechanism. (Note: There are Canon lenses that allow what they call Full Time Manual or "FTM", overriding of AF while it's operating... Most "USM" lenses can do this. But yours is not a USM or Ultrasonic Motor auto focus lens. So you must turn AF off at that switch, before manually focusing the lens. Feel free to zoom the lens, though. That will do no harm, but see below for more info about focus and zooming.)
So if you have been in the habit of manually overriding the focus of your lens without first turning off AF at that switch on the LH side of the lens... it's possible you have damaged the AF mechanism. Maybe this is why you had to replace your previous lens, if the AF failed.
Now, what's found on the camera body itself are several other things related to auto focus...
On the camera, you can choose the mode of auto focus: One Shot (for stationary subjects), or AI Servo (used to track focus with moving subjects), and AI Focus, which isn't really a focus mode at all... it's supposed to decide for you whether or not the subject is moving, then switch to use the correct mode. (I never use it, I decide which mode I want to use myself.) To change from one auto focus mode to another on your camera, you first press the button marked "AF" - on the back, RH side, about 2/3 of the way down (actually it's the "East" button of the 4-way controller) - then turn the dial on the top fo the camera to scroll through the modes and make your selection.
Also on the camera, you can choose the AF point or points (as seen in the viewfinder), that you want the camera to use to perform auto focus. You can choose to have the camera use all the points and hope it automatically selects the point of focus for you correctly, or you can select any single one of the points yourself (note: the center one is more sensitive, faster and more accurate on most Canon cameras). To do this, press the AF Point Selection button, which is on the back of your camera, far upper RH corner (also doubles to magnify the images when reviewing them on the camera's LCD, and is marked in blue with a little magnifying glass with a + sign inside). Then scroll through the individual AF points with the dial on top and/or using the 4-way controller buttons on the rear of the camera. When you scroll all the way to the edge, all the points should light up and you are back in "all points/auto selection" pattern.
The last thing related to auto focus on your particular camera is the shutter release button itself, which you are probably quite familiar with. When you half-press it, you activate or start AF (also IS and metering). Releasing the button fully stops AF. Halfway pressing it again starts it up again. If you are in One Shot mode (stationary subjects), auto focus runs until the camera senses focus has been achieved, then it stops, locks and gives you Focus Confirmation (green LED in the viewfinder and, if you have it enabled, and audible "beep"). If you have the camera set to use AI Servo mode, autofocus starts and runs continuously as long as you keep the shutter release button halfway pressed, tracking the moving subject and constantly updating. There is no Focus Confirmation in AI Servo mode because auto focus never stops and locks. You have to keep the selected AF point right on the subject, where you want the camera and lens to focus, continuously while doing this. If you have all points/auto selection set up, it is possible for a moving subject to be passed off from one focus point to another, which the camera displays in the viewfinder by lighting up the active AF point in red.
(Notes:
There is another technique of controlling auto focus called Back Button Focusing, which can be set up on most or perhaps all Canon cameras and is particularly useful when using AI Servo with moving subjects, though it also can be used with One Shot.... But I don't know if we want to complicate things at this point.
Also, some other Canon cameras have an "AF On" button on the back, that starts and stops the lens from auto focusing. Your camera doesn't have this button, but I wanted to mention it in case you see or hear reference to it elsewhere.)
Another key thing that can effect focus is that your 18-55mm is almost certainly a "varifocal" zoom. This means it does not maintain focus when you zoom and change the focal length (some more expensive "parfocal" zoom lenses do maintain focus). If you are using AI Servo it will automatically correct focus when you zoom...no worries. But if you are using One Shot, any time you zoom the lens you must re-focus afterward by lifting pressure off the shutter release button and reapplying the half-press. You should again get Focus Confirmation when you do this.
You might want to spend a little time viewing some online videos that do a good job explaining auto focus on Canon cameras. There are three videos, the first of which is at http://www.youtube.com/watch?v=iAx86nblZ2g. These three videos, each about a half hour long, talk about the auto focus systems on many different Canon models. Some things won't apply to your particular camera. But a lot of it applies to all Canon cameras and I think you might find it time well spent, viewing them.
Hope this helps!
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Alan Myers
San Jose, Calif., USA
"Walk softly and carry a big lens."
GEAR: 5DII, 7D(x2), 50D(x3), some other cameras, various lenses & accessories
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