Fiona, can you go into a little more detail? It sounds to me like you are helping a friend who wants to make a bilingual English/Chinese document in InDesign. Perhaps you're both college students? This is not very hard, so don't worry about the technical stuff for now.
So, has the Chinese text you want in ID already been written? You say that "I really don't understand how to input it." I can't tell whether you mean "My friend and I don't know how to type Chinese directly into InDesign," or "My friend and I don't know how to get this Chinese text we've already typed in Word (or Quark?) into Indesign." David covers both these possibilities, but it sounds to me like you will need step-by-step instructions for either typing Chinese into ID, or placing and manipulating Chinese text from a word processor into ID.
You won't need SimSun to do it, either. Besides, SimSun is - forgive me for being blunt - pretty ugly. If there's going to be any English text in your Chinese document at all, don't use SimSun. Adobe Song will be fine for simplified/mainland Chinese.