Select auto focus point - Canon EOS Rebel XSi Digital Camera

Are you sure you did not put the AF/ MF switch in the wrong position? Although I don't think it would focus in Auto mode. In all other modes, are you sure you selected a (or more) focus point(s) in the centre of the frame?
When the selected focus point is on the side of the frame it won't focus on the parts you want it to focus. Then while pointing to the point you want the camera to focus, please press the shutter release button half and what till it focusses. Then press it the whole way down. Be aware the camera needs enough light to focus and certainly can't focus with a lens wit a min aperture of 6 or a larger number. On your 18-135 you can shoot at f16 because it will first focus on wide open and only close the aperture after you pressed the button.

On the back of the camera top right you may see a button with an icon above showing 5 white blocks in a + sign in a white box. Press this and you'll see the focus points. Use the left / right and up / down button to select the focus points.

Your camera is a point and shoot. This means the camera does auto focus when you want to make a picture. The focus point is in the square in he middle of the screen. The only thing you can change is when you want to make macro pictures. The camera will try to find a focus point within 2 or 3 feet.
Ion some scenes, the focus will differ just a little, like in sports, it will try to be automatic.
The trick every professional photographer uses, is place the focus point on the part, you want to have in focus in your picture. Then press the shutter release button half and wait till the camera is in focus. Then reframe the picture as you want it to be. Then press the button fully. Just as described on page 31 of the manual.

Depending on your lens, you may have an AF/MF switch. Make sure it's set to AF for Auto Focus. Try increasing your light levels. Does focus work in a good light setting?

You should have a button that looks like a matrix. Press this button to set your focus point. Use the joy-stick like wheel to select the focus point. Press it to select Auto Focus points.

Try using manual focus. Make sure your auto focus points are correctly set. Look for button that looks like matrix. Press it then use/press joy stick type button to select auto focus point.

You may just have the left auto focus points selected. You don't say which camera model you have. If on the back of your camera you can see a button with an icon showing 5 small white blocks in a plus sign in a white box press this to select the auto focus points. It should show the available auto focus points and then use the left/right (wheel) button to change the selection

There are no rules or conventions for this. It is totally up to you to chose which method to use.

Sometimes when taking images in close up where a leaf or stalk may invoke autofocus to lock onto the wrong object to get a clear pic, then select manual so your subject is in focus

Depth Of Field is also associated with focusing, autofocus does not always deal well ith DOF situations. It might be better for you to get a simple book on digital photography to help you understand how to get the best results from your camera

Please rate my help++++Thanks for using FIXYA

Try checking C.Fn III, 3: AF point selection method. Yours might still be on option 2.

0: Normal - Works by pressing the AF Point button and then the Multi-controller.

1: Multi-controller direct - Press the multi-controller to directly select an AF point (without first having to press the AF Point button). Pressing the AF button reinstates all AF points for auto AF point selection.

2: Quick Control Dial direct - turn the Quick Control Dial to select AF points. In this mode, AF points will not be displayed on the LCD panel.

There is a bar that transfers the movement of the focusing ring to the motor in the back of the lens.  If it jumps out of the plastic slot, you will hear the AF motor turn, but it will never focus in either auto or manual mode.  You will have to buy a new lens, get the lens repaired (probably more expensive than buying a kit lens), or partially take the lens apart and re-seat the bar.

Here is a link showing how to fix it:
http://forums.dpreview.com/forums/read.asp?forum=1022&message=17650836

I did it successfully in about an hour with a set of jeweler's Phillips screwdrivers, tweezers and a strong lamp.  You MUST be comfortable delecate parts and tiny screws.  Then again, you have nothing to lose if you are going to buy a replacement lens.

Normally the camera measures the distance to the subject using the 3 auto focus frames (left, center and right auto focus frames) in the viewfinder and selects the most appropriate point automatically. You can also select any one of the three points.