Currently, the only kick that seems safe for the kicker to me, is a side kick with the heel or the balls of the feet. A kick with the back of the feet, like I see in taekwondo is really only for the head right? Otherwise if I kick something that doesn't move, then something below the shin area, I know only from experience, gets pulled hard and it hurts for days. Also, my friend showed me some videos of professional fighters kicking with the shin and accidentally hitting the opponents knee, resulting in the kicker's shin being broken into two, which is a major no-no for cowards like me. With which part of the leg do I kick, and when I kick with that part, where do I want it to hit?
From my years of ITF TaeKwonDo.
A turning kick should impact with the ball of the foot - it is important to straighten the ankle and pull the toes back. The attacking tool is important - because we walk around all day on our feet the ball and heel are conditioned to the impact already.
This does mean that the opponent is more in-front of you than most beginners think. (think diagonal forwards - not to the side). We can still attack opponents with this kick who are not in this position by turning our body - this act can add additional power if it is in the same direction as the kick.
As for the target areas >> most vital spots are acceptable targets and this will depend on what is visible/accessible and how flexible you are. Knee/Groin/Abdomen are typical targets (none sparring) - anywhere in the body with lots of nerves close together - or a weak joint.
In sparring it is common to use the top of the foot - this allows a little more range. However this is sacrificing power/damage (which sparring isn't really about anyway) and should obviously be kept to scoring targets (not vital spots).
Hard on soft is a good general rule.
The hardest parts in the human body are the ones with a bone directly supporting it. For example take a fist vs the heel of the palm. The palm aligns directly to the bone whereas the fist has many small bones in the way. This is why it is easier to break the bones in the hand when punching unless they are well conditioned.
Knowing this then you can understand that the heel of the foot when thrust in line with the bone will be harder than the toes or the ball of the foot (unless of course there is conditioning).
The shin has no supporting bone and impacts with a shear force (which is more damaging) and relies on momentum. To avoid fracture it needs to be conditioned to be effective or hit on a soft part only. Knees are defintely not soft.
So using the heel aligned to the tibia will require less conditioning than using the other parts of the foot. The best method of using this part to kick will be to thrust in. So the kicking options are front, side, rear or a spiraling mix of these.
The target is important to apply hard on soft. For a front on opponent the targets are the face, solar plexus, bladder and groin. With a little conditioning though these thrust kicks can be applied on the chest, thigh, knee, shin and foot with a lot of effect.