help 550d focus problem
You may be selecting a larger aperture (smaller f-number like 1.8, 2, 2.8, etc.) and taking photos with shallow depth-of-field.
Set your camera to Aperture Priority, then dial in an f-number of 8. If the subject still isn't completely in focus with that setting, try f/11.
As for your focus point: once you use f8 or f11, then it shouldn't matter which focus point you use as long as that point is on the subject. If you're filling the frame with your subject anyway, you may as well just use the center point.