Disapparating and Apparating are skills you learn when you are sixteen or seventeen. You are only given your license to legally do it when you are seventeen. 

Although Apparating and Disapparating is fun and a good way to get to class on time, you can not Disapparate or Apparate in Hogwarts grounds, although the enchantment preventing you from doing so is lifted for a brief period of time while the students are taught, don't try it when you aren't learning. 

Splinching is a common side effect of Apparating or Disapparating. Splinching is when you leave a body part behind after Apparating or Disapparating. It is quite painful, but happens to many people, especially ones without much practice. Most people leave behind only their eyebrow or a fingernail, but sometimes much bigger, more important things are left behind, like arms or legs. You can go back to where they left and reattach them magically.