what should i do with my newly build pc first time

Toss it out because it was the practice Pc. Now buy yourself a really good one, build it and keep it.

Sorry, sometimes I can't keep those thoughts in. Is it built and you need to install windows? That's kinda what it sounds like....
 
First thing you need to do is to install an operating system. Windows is very popular but others are available. It all depends upon what you intend to do with the computer.