The long pvc
pipe that is currently connected is your return pipe which is connected to that pump. The other hole needs another pipe and will be your drain pipe Both of these pipes will have to go to a sump.
Take a look at the lafishguys.com
and watch all the parts of Episode 51, Setting Up A Reef Tank. You will see how the tank gets hooked up to the sump from start to finish. You don't have to set it up this way, but will get the general idea.
Yikes I would drain that tank. You are not ready to get it setup yet.