help 550d focus problem

i have a problem with my 550d,when taking a photo with my 50mm 1.8 lens and filling the frame with the subject,when i press the shutter half way the focus point always seems to be on the wrong part.the little red dot always is at the bottom, which means the bottom is sharp and the top is out of focus.i know i can pick which focus point i want but i want the whole subject in focus.does anyone know how to do this?