Depends on what type of coral it is, and what its light and flow requirements are. Higher light requirement needs to go up top closer to the light, lower light needs to be down closer to the sand. There are lots of different ways to mount corals (I typically use superglue) but it really depends on what it comes attached to. And yes, you can place it just about anywhere as long as the light and flow requirements are met. Also keep in mine space for growth, expansion, etc around other corals. Take a look around at the tanks in this forum and you will get some ideas.
HTH