Lol... that happens!
Well usually.. for AF each DSLR uses an AF sensor (Its a CAM Sensor in the XSi) and the sensor only receives light when the mirror is down!
For Live View the main Sensor (the one which absorbs the light to take a picture, it remains fixed behind the mirror) must be exposed at all times but its not so efficient at AF. So Canon gives you two options:
1. Pull mirror down, pause Live View, AF and pull mirror up! This is comparatively faster since the focusing is still done by a specialized AF Sensor. -> Phase AF|Quick Mode (since its quicker)
2. Use the Main Sensor for AF meaning mirror interruption not required so constant Live View. But since the sensor is not specialized for AF.. it uses what it can do best: detect light. So it uses a contrast based AF detection (contrast is the difference b/w light and dark) and also takes a very long time! It is also unreliable in case of low light or bright light since contrast at these times is very low! -> Contrast AF|Live Mode (since Live View remains on)
To Enable the Contrast based Live View, In Menu, go to the third settings option (the spanner icon with three dots). Scroll to "Custom Functions" and select it using the 'Set' Button.
Now use the side Keypad buttons to scroll and come to something called "AF During Live View" (Its probably the 7th Custom Function.. probably)
Select it using the 'Set' Button and scroll down to the "Live Mode" option and select it using the 'Set' Button again.
There.. you're done!
Press menu to return and enjoy!
But let me be frank.. avoid Contrast AF since its terrible and you have to squiggle the small box around the whole screen to select an AF Position!
Also... DECREASE YOUR DEPENDENCY ON LIVE VIEW. Dslrs are very heavy and using them stuck to your face and squinting eyes makes them much more stable. Also, Dslrs have huge HUGE sensors as compared to pns... meaning Live View in DSLR will be one big battery drain as compared to point n shoot (pns). Third.. you'll come to realize that a DSLR is best used with the viewfinder. Just practice using the viewfinder and it'll seem comfortable!
If you ought to use Live View in positions very uncomfortable like above your head or near your feet... manually select the AF point and use the quick mode AF!
Finally.... after a terribly long post (Sorry).. your manual is your best friend. Read it from the beginning to end and dont skimp anything. If you still have a problem.. we here at MR are here.. just fire up a thread or PM me
Apple Ink