Why do you want to do that? Is it because of disk space shortage on C: ? Then I would advise you to resize the drives. Shrink D: and enlarge C:. There are utilities which can do that for you, like Acronis Disk Director or Paragon Parition Manager (both of them not freeware), or Ranish Partition Manager/NTFS Resize (these being freeware, but I never used them,). If you Google for NTFS partition resizing software, you'll get a lot of hits. On tip here: when shrinking the D: drive, make sure you reduce it from its beginning, not from its end, so you make room for a bigger drive C:.
Switching the system drive from C: to D: involves extensive registry hacking and since you yourself said you have limited computer knowledge, messing with the registry to such an extent would be disastrous.