Focus problems... HELP!

-0- Depth of field : get some calculation tables if your camera have no dof button, sharp zone (focused) is 1/3 before the focus point, 2/3 behind the focus point roughly , but it depends of aperture of cause. Study this relation until it is in your blood and you dont have to think about it.
-1- type of focus :
Multifocus focus on the nearest object in sight, like if you use multifocus on a frontal portrait with a very short dof (aperture wide open) you focus on the nose, not on the eyes. This is more suitable for general photography.
If you camera allow single focus on several places, (a little square you can move on your viewfinder), place the focus to the nearest point to where you want to focus, half press : the camera will show you where the focus point will be (just around).
Imy nikon camera make the square blink and show the place).
This can be coupled with tracking for mobile objects, and is great for sports, animals, shows...
If your camera offer the above, it probably offer a way to "lock" the focus on a place, then you reframe.
I have another camera than yours, so study the manual to find out the buttons !
It is suitable for portraits, because in real situations the time to reframe is often too much.
If your camera have only one centered point of focus, and you still want to use autofocus, the locking is usually done by half pressing the shutter button.
Some lenses does not allow manual focus, just like old lenses doesnt allow AF. If your young eyes are still good, and you are quick, you would be surprised to realise how manual focus is easy, even easier than AF in shallow light. Try it and the focus will be really yours.
2-lens problem
This can happen. But if this happen with all you lenses, it
3-sensor problem ! Some Canon camera have this defect in some series. Google with "back focus canon"

You have to make methodic tries, even if it is boring. I suggest a tripod, a note sheet, and a graduated meter.
Hope it helps