As already mentioned above: shin conditioning. Let me add some words, too.
You should start on a soft bag, then continue with hard bag, and then - with roped logs (you may see it in your gim - roped log, wrapped with rope). Roped logs are also differ - usually, the thinner is wrapping rope, the harder is to kick it.
The key points in conditioning - regularity and amount of kicks, hardness will come later. You should not kick with all your power. Soft kicks, but with big amount. You may start with, for example, one hundred kicks for each leg once a day. Then increase amount, and when you reach 5 hundreds, you surely may move to the next bag.
Let me also give you some warn. Please, don't use common trees/wall corners as a bag alternative. You may see in some movies, that real thai fighters (or Jan-Claud Van-Damm) do exercises with palms. The fact is, that they do it on special palms - their softness is close to wrapped logs in your gym.
General rule is to use things, that are not more solid than your leg.
For shin kicking vs foot lift kicking - as mentioned above - shin kicking is much less trauma-causes - just because there are much less bones.)
Good luck and make your shins metal!