Just as Mark and i thought, youre not actually focussing on your intended subject but youre just centering your subjects in the frame and hitting the shutter, the focus is hitting whatever is in the centre iof the frame, you need to track your subject with the centre AF point
Lesson No1, put the camera away and dig out the manual and read it from front to back several times, you need at least a basic understanding of how your camera and its AF system, tracking and AF expansion/assist points works before you pick it up again.
Please for now dont get sidetracked into buying a new lens, theres absolutly nothing wrong with what youre using for the moment, whats at fault is you and your understanding and technique, a new lens will only make whats currently in focus a bit sharper and whats out of focus much softer due to the narrower depth of field and this will frustrate you even more.
Dont buy off Gumtree unless you want a good kicking and your cash pinching.
All this may seem pretty harsh Gaz but from your questioning regarding how to choose your AF point and other questions its blatantly obvious you need to go right back to basics, you obviously dont understand your equipment, how it works, how to use it or its capabilities.
I think you need to join a camera club and get some one on one tuition and READ THE MANUAL SEVERAL TIMES OVER.
We can talk you through some things but i woukld suggest that you would be better with one on one.